March 15, 2026

Stellar Evolutions, Dark Energy Mysteries & Your Questions Answered

Cosmic Q&A: Red Giants, Accretion Disks, and Dark Energy In this captivating Q&A episode of Space Nuts, hosts Andrew Dunkley and Professor Fred Watson tackle a variety of listener questions that span the cosmos. From the fate of our Sun as it...

Show Notes
Transcript

Cosmic Q&A: Red Giants, Accretion Disks, and Dark Energy
In this captivating Q&A episode of Space Nuts, hosts Andrew Dunkley and Professor Fred Watson tackle a variety of listener questions that span the cosmos. From the fate of our Sun as it becomes a red giant to the mysteries of dark energy, this episode is a treasure trove of astronomical insights and engaging discussions.
Episode Highlights:
- The Fate of Our Sun: Jeff from Arkansas asks about the implications of the Sun swelling into a red giant in approximately 5 billion years. Andrew and Fred explain the process and its potential effects on the outer planets, addressing concerns about rogue planets and gravitational influences.
- Understanding Accretion Disks: Blue from London inquires about the apparent high-speed motion of material in accretion disks around black holes despite gravitational time dilation. The hosts clarify the dynamics at play and the distances involved in these cosmic phenomena.
- Expanding Universe Mysteries: Julian from Canada poses questions about the expansion of the universe and its acceleration. Andrew and Fred dive into the complexities of dark energy and the Hubble constant, shedding light on current theories and ongoing research.
- Dark Energy and the Multiverse: Peter from Sandy Kaye explores the possibility of unseen matter in the universe affecting expansion and whether other universes could influence ours. The discussion delves into speculative theories and the nature of gravity.

For more Space Nuts, including our continuously updating newsfeed and to listen to all our episodes, visit our website. Follow us on social media at SpaceNutsPod on Facebook, Instagram, and more. We love engaging with our community, so be sure to drop us a message or comment on your favorite platform.
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Stay curious, keep looking up, and join us next time for more stellar insights and cosmic wonders. Until then, clear skies and happy stargazing.

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WEBVTT

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Hi there, thanks for joining us. This is a Q

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and A edition of Space Nuts. We talk astronomy in

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space science, and in the Q and A episode we

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answer questions from our audience. We've got a whole bunch. Today.

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We're going to hear from Jeff, who wants to discuss

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the effect when the sun goes red giant, which is

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going to happen in a couple of weeks. No, No,

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it's not blue. Is asking about accretion discs, Julian about

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the expansion of the universe, and one of our old

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favorite topics from Peter, dark energy. That's all coming up

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in this episode of Space Nuts. Fifteen seconds guidance in

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Channel ten nine ignition sequence Space Nuts NY four three

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two more red one Space Nuts as the night record it.

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Bill's good.

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And back again to furnish us with his wisdom or

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just to have a couple of lucky guesses. Is Professor

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Fred Watson, Astronomer at Large. Hello, Fred, Oh you doing,

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Andrew good to see you good and you yes.

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Sir, firing on all cylinders as far as I.

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Know, that's all right. Back then they only made them

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with three sel Oh gosh, I mean wicked mood today. Sorry,

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about that. Yep, Okay, we better get on with it now.

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I will put it out an appeal like I did

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at the end of the last episode for audio questions.

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Questions in general, but audio questions particularly. We are desperately

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short and we don't know why. It's just a weird

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quirk of fate, I suppose. But if you would like

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to send us some audio questions, you can do that

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on our website space nuts dot io and click on

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the AMA ask Me Anything tab at the top and

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send us the message. Don't forget to tell us who

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you are and where you're from. Our first question, Fred,

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comes from Jeff in Fayetteville, Arkansas in the United States. Hello, gentlemen,

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Oh he's wrong for a start. Come on. I happy

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upon your podcast several months ago and enjoy it very much.

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Last month, you discussed the thought experiment having to do

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with what would happen if the rest to the rest

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of the Solar system if the Sun suddenly disappeared. My

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question is a little bit more concrete. In five billion

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years or so, our Sun is expected to swell into

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a red giant before perhaps collapsing into a white dwarf.

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What will this do to our son's total mass and

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what effect will it have on the outer planets that

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survive being engulfed, Earth not being one of them, as

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it turns out. A follow up question, are other sons

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dying throughout our galaxy responsible for the rogue planets far

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out in space between stars that I've read about. Thank

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you for your time and the stimulating discussions you allow

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me to listen into. Thank you. Jeff's lovely a few

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questions in there. Fred. So, Yes, our sun, which we

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have talked about, going you know, ballistic about five billion

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or so years, what happens to the outer planets?

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That's yes, it is a great question, and I think

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the sort of implicit assumption is that is that the

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inner planets are not going to be around, which probably

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will include the Earth and possibly even Mars.

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So the.

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Yes, the process towards the end of the Sun's life

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and somewhere in the region of three to five billion years.

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I think the process starts about three billion years hence,

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so it's a little bit earlier than Jeff says, so

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that's not a cause for alarm.

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Three billion leads years.

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That he's watched, so he's going to have to rese

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it back and that's alway. The pain is such a right.

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Yeah.

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I the other day set dutifully set the timer to

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time down on my phone because I had to take

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things out of the oven, and it got to the

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end of the time and didn't do anything. Just how

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you stopped.

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I did too.

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My phone suddenly decided to put all calls through the

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voicemail before I can even before I even know it's ringing.

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Picked it up the other day and thought, why is

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it vibrating? And then I realized my sister was ringing me,

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but it was making no sound. There was no indication.

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It just filtered, And when I pushed a couple of

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buttons eventually found her and she said, ah, yeah, I

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was just listening to your message. There you go voicemails,

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which is not a problem.

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Will be really worried about intry, not just time.

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But if anyone knows the answer to that, please let

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me know.

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Well, I can tell you the answer. You have a

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smart watch and the wings as well.

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Well, yeah, I don't think I had it on at

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the time.

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Anyway, So so yes, the inner planet's get swallowed up

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as the sun swells. It doesn't happen suddenly. It is

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as a leisurely process. There is a collapse of the

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core down to a white dwarf star, which is the

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end product. Is this white dwarf star, which is the

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size of the Earth, but with quite a lot of

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the mass of the Sun still tucked in there an

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outer shell of expanding gas, and if we were looking

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at that from the outside, we'd call it a planet

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chair nebula. So that outer shell of gas is hot.

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It's hot enough to basically vaporize the inner planets.

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But Jeff's questions not about.

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That, it's about how do things look from the outside,

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and so the outer planets. Interestingly, their orbits will be perturbed,

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which means changed, but maybe not as dramatically as you

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might think because that swelling. What it's doing is if

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you look at the center of gravity of all that,

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it's still at the center of the Solar System, even

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though the outer layers of the star and a significant

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amount of mass in that. Even though most of the

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mass are going to be concentrated in the white dwarf star,

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but the outer layers are expanding, you're outside that zone.

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So to you, the center of gravity, center of mass

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of the Solar System remains where the sun is now,

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and so the effect on the orbits of the planets

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might not be as dramatic as propelling one of them

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out to become a rogue planet. So it could be

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a lot gentler than I mean, a super and ova

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is different. If it explodes, then you're talking about dramatic

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and cataclysmic events that would certainly disturb the orbits of planets,

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although some planets seem to be able to survive that.

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We can see supernerova eminence with planets chugging around anyway, this.

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Scene.

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The scenario is that there may still be planets which

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will be outside the you know, the envelope of the

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of the white of the red Giant. Now, eventually that

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envelope is going to pass the past the outer planets

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and may evaporate them as well, depending on what sort

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of temperature it is. But the so the red giant phase,

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the outer planets might be still okay and still say

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it's stable. But once it progresses beyond that, and you

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get to the planetary nebulus stage, where the outer layer

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is light years in diameter rather than rather than just

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a few hundred million kilometers in diameter, truly in kilometers

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in diameter. Then you're you're you're probably in trouble if

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you're on the outer planets as well.

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Okay, how long does that process take to reach that

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nibula stage.

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It's it's relatively slow. You're talking about billions of years. Yeah,

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so I mean there must be there must be a

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collapse phase for the nucleus. I'm not an expert on

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these what we call highly evolved stars, but as it

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goes from red giant to a planetary nebula, that's when

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you change into a white dwarf. In the middle, you've

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got the nucleus of the star collapsing. So these are

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the end products of normal stars. Actually, a normal star

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will go through this phase, a massive star will become

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a super and ova will explode.

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Wow.

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Okay, so you pretty well answered everything in one hit.

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The white dwarf will have the same mess as our

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Sun at the moment, more or less, so it won't

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really have an impact on the out of planets until

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after the halo effect for billions of years, and probably

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not enough effect to cause rogue planets.

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I think that's right, Yes, I mean I think if

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operated planets is more likely, so rogue planets are most

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likely formed either just formed in gus clouds collapsing into stars.

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These are things that are big enough to collapse into

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stars so they become little planets on their own, or

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by being ejected because of a gravitational influence. You know,

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if you get two planets that pass very close to

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each other or something like that, one of them might

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get booted out of their solar system.

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There you go, Jeff, Hopefully that answered all of your questions.

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We like multifaceted questions. It text us in all sorts

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of strange directions. Sometimes great to hear from you. Hope

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all is well in Arkansas. Fred. Our next question comes

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from Blue is from London. I have a question about

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accretion disks. Given that gravitational time dilation causes time to

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pass much slower near a black hole relative to Earth,

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why does the disc appear to move at high speeds

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to us? I would have expected the material to look

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like it's moving at extreme slow motion or even stop

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moving the closer you get to the event horizon. Love

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the show, guys. Thanks Blue. Okay, now you're going to

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know the answer to this one, because I think we've

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had a similar question before, But it seems to be

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an ongoing theme with black holes. Why do we see

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things that we don't think we should see.

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So, yes, the accretion disc is actually quite a long

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way from the black hole, and a lot of that

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swirling material is ejected from the black hole rather than

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getting sucked in by the magnetic fields that the black

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hole generates. It sort of projects the accretion disc material

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at very high velocities to the sort of north and

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south of.

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The accretion disc.

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So there will be a time dilation effect.

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I should check this as to how much it would.

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Be, but I think the overarching point is that far

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enough away from the event horizon that time dilation in

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the accretion disk is not great. It's not high enough.

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I think it's all a matter of scale rather than

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being a matter of the phenomenon, which I think Blue

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is right. I think is you know, anything that gets

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close to the to the black hole's event horizon is

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going to show up a time dilation phenomenon.

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I suppose it would also depend on what kind of

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black hole it is. I mean some of them are

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massively Yes.

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That's right, that's correct.

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So a super massive black hole will produce a bigger phenomenon.

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I'm just going to try looking something up to see

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if I can put any detail on that.

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I'm helping you a lot here.

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Yeah, okay, all right, here you are. Here's a number

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for a ten thousand and solar mass black hole, and

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that's a bit. I mean, that's the kind of limit.

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Of what we call an.

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Intermediate mass black hole, something measured in tens of thousands

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of solar masses. For a ten thousand solar mass black hole,

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the time dilation of the increase the accretion disc inner

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edge is only twenty two percent. So it's what I've said,

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it's the scale of the process that makes makes the difference.

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Fair enough, okay, simple answer.

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Really, yeah, it seems to be.

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All right, Thank you Blue. Hope you're going well. Love London.

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Only been there twice I think twice, yes, and yeah,

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I really enjoyed my time. It's a beautiful city. Really

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love that place. So I'd go back tomorrow for could.

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Thank you Blue. And this is space Nuts. You're listening

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to it with Andrew Dunkley and Professor Fred what's an.

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We choose to Golden Moon and this decay and do

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the other thing not because they are easy, but because.

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They are are these nuts. Next question Fred hey nutters

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Julian from Brampton, Ontario, Canada. Here, what is causing the

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expansion of the universe? Oh? That old chestnut? I mean,

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are we a black hole eating outside matter? Or are

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other fundamental forces responsible? Or are the forces actually keeping

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us together? Part two? I think we're up to part four. Actually,

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part two of this question would be Fred mentioned the

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universe is expanding slower. Does that mean we know exactly

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how fast the universe is expanding? And does that mean

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speeds faster than light? I hope you all got that.

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Love the show, thanks, Julian. Oh, he's packed in a

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lot there in three or four sentences. What's causing the

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expansion of the union of a spread? And I know

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the answer to this one.

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I think, well, you know, the way we usually interpret

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the Big Bang is in the beginning, there was nothing,

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and then it exploded and that's kind of more or

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less what it was. And so we're still seeing the

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aftermath or the effect of that explosion in the expansion.

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So it's driven by by a you know, some fundamental

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injection of energy at the start of the universe. We

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can kind of quantify how much it was. It's a

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very large number that set the expansion in motion, and

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it's still going on now. The second bit of the

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question is the is the interesting bit part two? So

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so let me just yes that there are of parts here.

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Are we a black hole eating outside material? Some scientists

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have suggested that we are within the event horizon of

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a brook ball, and that's what I've heard what's causing

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the expansion of the universe. It's a very speculative idea.

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It's not one I'm very fond of because I you know,

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I think I think we need a bit more hard

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evidences to that black hole's there. Are we a black

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hole eating outside material? Probably not?

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Or are there other fundamental forces responsible?

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Well?

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Yes, maybe, And that comes to the second bit of

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the question. It says Fred mentioned the universe is expanding slower.

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That's actually not the case. It's expanding faster all the time.

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But that acceleration is reducing, reducing. Yes, so the exposure of.

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Conundrum, isn't it right? It is us expanding at a

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faster rate. Now it's expanding faster, slower.

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Faster, slower, that's right. I mean, before we used to

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think it was expanding at a slower rate. We used

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to think in the seventies and eighties that it was

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slowing down, the expansion was slowing down, and one day

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they'd be perhaps a reversal of the expansion and a

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big crunch. Then in nineteen ninety eight, along Cane, Branchmidt

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and saw Perl Muta, and between them they figured out

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and Adam Reice, of course the three Nobel Prize winners

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in twenty eleven, they figured out that actually the expansion

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is accelerating. And that's still the case today. That is

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the situation we have. The expansion is still is accelerating,

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but there is now new evidence that suggests that the

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acceleration is reducing. So it's not that the expansion of

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the universe is slowing down. It's that the acceleration of

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the expansion of the universe is slowing down, and that's

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a different matter. So it is still it's still accelerating

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in its expansion, and we tribute to dark energy. That

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is something that the new results that suggest that the

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acceleration is slowing down actually throw a spanner in the

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works because we thought we understood dark energy as what's

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called a cosmological constant, a force that basically is proportional

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to the size of space. The more space you have,

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the more expansion force. There is a kind of pressure,

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if you like, off space itself springing us of space.

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The fact that as time goes on that expansion seems

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to be the acceleration of the expansion seems to be reducing.

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Is it's actually suggestive of new physics, which might mean

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higher dimensions and all the other good stuff that we

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love talking about on space nuts.

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Yes, indeed.

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So his last bit of the question, does that mean

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speeds faster than light? There will be a limit beyond

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which we can't see the universe because it's expanding from

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us faster than light can get to us. That's already

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the case, but we don't see that because we see

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the cosmic microwave background radiation first.

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That's the nearer phenomenon.

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So yeah, So it's an interesting set of questions which

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I guess highlighting some of the mysteries that we face

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with with our current understanding of cosmology.

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In truth. In truth, though we do not know how

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fast the expansion rate currently is. We can't put a

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number on that, can we?

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Yes, we can.

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Okay, that's the Hubble constant, which is, yeah, we have

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we talked about that before. We've yeah, because because there's

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this tension that between the Hubble constant as we measure

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it now and the Hubble constant as we determine it

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from the cosmic microwave background radiation called Hubble tension, which

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may be something that really is just to do with measurements.

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So the Hubble constance in odd units in it's in

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kilometers per second per mega parsek, which is basically a

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fancy way of saying we do no health.

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LASTI sex badis Yeah, the answer is seventy to seventy

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four kilometers per second per megaparsic.

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Yep, give a take. Well that's right.

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You know, when I was a young astronomer keep saying

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this back in the seventies, there were two The estimates

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of that number were out by a factor of two.

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There was one group said it's fifty kilometers per second

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per mega parsec and other group said it's one hundred

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kilometers per second megaparsec. And look what we've got with

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the Hubble telescope and other instruments. We've got basically the

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average of those two, which is quite neat really, but

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we still got some uncertainty.

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There was also this conflict, wasn't they that they were

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I couldn't figure out the answer for. But we did

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a story about them, figuring out that the conflict was

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actually normal, and we you know, that's within acceptable parameters.

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I think it did. Yeah, Okay, do we answer all

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this question? I think we did. Yeah. Oh, I did

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look up something else. How powerful was the Big Bang?

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Greater than ten to the sixty eight rules?

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Okay, that's what you call a bag.

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That's a big, big, big, big, big big bang. But

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we just shouldn't it a big bangs? Good number?

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Turned to the sixty eight duels?

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Yes, yeah, put that on your calculator and see what happens.

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Thank Louid Julian, I hope you will. Let's answer one

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more question before we finish up today, and this is

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a question with forty seven parts. Hi Professor Fred and

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Andrew Peter here from San Diego, California. I have a

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dual question about dark energy. Under standing is that dark

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energy is causing our universe to expand also, we have

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an observable part of the universe where light is able

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to reach us, and an unobservable part of the universe. Funny,

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have we already talked about this, but all these questions

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kind of dovetailed. Question one, is it feasible that the

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unobservable part of the universe contains enough matter to act

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gravitationally on the observable universe, thus causing the expansion? And

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question two, which we'll get back to, same question. Within

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the context of the multi sphere, could universes other than

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our own be gravitationally impacting the expansion of our universe?

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Love your show, keep up the great work, Thank you, Peter.

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Question one, is it feasible that the unobservable part of

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our universe contains enough matter to act gravitationally on the

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observa observable universe, thus causing the expansion.

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Yes, once again we've got to unpick what we mean

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by the explod So Peter says, my understanding is that

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dark energy is causing our universe to expand, and that's

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not the case as we've just described. It's the Big

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Bang that caused the universe to expand, but dark energy

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is what's causing the expansion to accelerate to get faster.

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It's EASi it confused now, isn't it, Because it is

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absolutely not. This is an event with a lot of

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moving parts.

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Yes, as you'd expect. It's a universe after all.

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But what Peter says is correct because we, as we've

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just said, there are you know, there are parts of

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the universe well beyond thirteen point eight billion light years

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which we can't see because what we run into is

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the cosmic microwave background radiation the flash of the Big Bang,

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So we know there's universe beyond that.

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But we can't see it.

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And Peter's suggestion is it feasible that the unobservable part

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of the universe contains enough matter to act gravitationally on

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the observable universe and thus causing the expansion. It's actually

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thus causing the acceleration of the expansion. That is one

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of the theories that people have looked at exactly that

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that are we in a kind of local bubble where

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the density is low compared with what is outside the

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boundary that we can see where the density might be higher,

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and hence having a gravitational effect on our local bubble.

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It's certainly a you know, a conjecture that is raised

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by by astrophysicists and cosmologists.

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So pieces on the right.

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Track there and then question two, the same question within

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the context of the multiverse. Could universes other than our

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own gravitationally be gravitationally impacting the expansion of our universe?

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And I think that is also something that's considered. One

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