The universe is expanding faster than expected • Hacker News Board

Re: The universe is expanding faster than expected

#231

Post by odorousrex » Tue, Dec 22, 2020, 8:19 PM UTC

tokamak-teapot wrote: post #212

throwaway201103 wrote: post #182

Earlier quoted context omitted.

If you subscribe to Roger Penrose's theory, at the point of infinite expansion, the current "Aeon" will end and a new big bang will happen. As best I understand it, at the point of infinite expansion, there are only photons. Mass no longer exists therefore scale no longer exists. So an infinitely huge universe becomes indistinguishable from an infinitely small universe.

What causes the Big Bang in this scenario?

Basically, the way I understand it, is the probability of a Big Bang spontaneously occurring (due to the quantum fluctuations) is some godawful infinitesimally low number 0.00000...1 that for all intents and purposes is 0, but is NOT zero.

So given ungodly eons of time (googleplex ^ googleplex * graham's number, etc. of years) something with a very, very, very low probability becomes inevitable.

Re: The universe is expanding faster than expected

#232

Post by pdonis » Tue, Dec 22, 2020, 9:10 PM UTC

cygx wrote: post #199

pdonis wrote: post #129

Earlier quoted context omitted.

> the most distant astronomical object we've observed has a redshift of 11 And even though that seems like a large distance , it is a very small redshift compared to the total time the universe has existed. The cosmic microwave radiation background, for comparison, was emitted at a redshift of about 1100. So a redshift of 11 only covers 1 percent of the expansion of the universe since the CMBR was emitted. > we've se…

No, we haven't. See above. No we have. See above: While redshift goes all the way up to infinity, in terms of cosmological time, there's still only 0.4 billion years between z=11 and z=∞, or about 3% of the age of the universe.

> there's still only 0.4 billion years

In terms of time, yes. I was thinking in terms of observable spacetime volume. Even though there's only about 0.4 billion years before z = 11, the scale factor increased by a huge factor during that time (a factor of 100 from CMBR emission at z = 1100 to z = 11). That's a lot of spacetime volume from which we have no useful observations.

Re: The universe is expanding faster than expected

#233

Post by pdonis » Tue, Dec 22, 2020, 9:29 PM UTC

gpderetta wrote: post #219

pdonis wrote: post #177

Earlier quoted context omitted.

> Turns out the math was right. Actually, turns out the math was right with a cosmological constant: that's how the accelerating expansion of our universe is explained. Einstein actually blundered twice : first by putting in the constant for the wrong reason (because he wanted a static universe), but then taking it out again when it turned out that reason wasn't the case (when the expansion of the universe was discov…

Re: accellerating expansion, I was only commenting on how the expansion itself is a consequence of GE. Thanks for your point about the constant being actually required. I do not understand the math, but is this similar on how integrals always have a constant as a free parameter that need to be determined by other means? I assume that Einstein originally set the constant to exactly balance the expansion, but later set…

> is this similar on how integrals always have a constant as a free parameter that need to be determined by other means?

Not really, no. It's a consequence of the assumptions made when the Einstein Field Equation (EFE) is derived from a Lagrangian using the principle of least action.

The assumptions are that the Lagrangian should be a Lorentz scalar (which is required of any Lagrangian) and that it should include no more than second derivatives of the metric. The Ricci scalar R meets this requirement and is the Lagrangian that was originally used by David Hilbert to derive the EFE (without a cosmological constant). But a simple constant (the cosmological constant) also meets the requirement, and therefore should be included in the Lagrangian; including it leads to the cosmological constant term in the EFE.

Re: The universe is expanding faster than expected

#234

Post by pdonis » Tue, Dec 22, 2020, 9:33 PM UTC

gpderetta wrote: post #219

pdonis wrote: post #177

Earlier quoted context omitted.

> Turns out the math was right. Actually, turns out the math was right with a cosmological constant: that's how the accelerating expansion of our universe is explained. Einstein actually blundered twice : first by putting in the constant for the wrong reason (because he wanted a static universe), but then taking it out again when it turned out that reason wasn't the case (when the expansion of the universe was discov…

Re: accellerating expansion, I was only commenting on how the expansion itself is a consequence of GE. Thanks for your point about the constant being actually required. I do not understand the math, but is this similar on how integrals always have a constant as a free parameter that need to be determined by other means? I assume that Einstein originally set the constant to exactly balance the expansion, but later set…

> I assume that Einstein originally set the constant to exactly balance the expansion, but later set it to zero.

Einstein didn't include the constant at all in his original equation, published in 1915.

In (IIRC) 1917, he realized that his original equation did not allow a static solution for the universe as a whole. He also realized that including the cosmological constant term in his equation would be mathematically valid, and that if he picked just the right value for the constant, he could obtain a static solution for the universe. At that time, it was generally believed that the universe was static on large scales.

Then, later, when it was discovered that the universe is expanding, Einstein dropped the cosmological constant term. He later called including that term in 1917 "the greatest blunder of my life", because if he had just gone with his original field equation, without the constant, he could have predicted the expansion of the universe more than a decade before it was discovered.

> In bot cases you are picking arbitrar values

When the expansion of the universe was discovered, yes, it was already recognized that it is valid to include the constant in the Einstein Field Equation, so it couldn't just be un-included. Its value was just assumed to be zero since that was consistent with all observations that were known then.

In Einstein's original 1915 field equation, however, the constant wasn't "set to zero". It wasn't included at all; nobody even realized at that time that it was valid to include it.

Re: The universe is expanding faster than expected

#235

Post by pdonis » Tue, Dec 22, 2020, 9:38 PM UTC

cygx wrote: post #222

pdonis wrote: post #177

Earlier quoted context omitted.

> Turns out the math was right. Actually, turns out the math was right with a cosmological constant: that's how the accelerating expansion of our universe is explained. Einstein actually blundered twice : first by putting in the constant for the wrong reason (because he wanted a static universe), but then taking it out again when it turned out that reason wasn't the case (when the expansion of the universe was discov…

> If you just look at how to derive the Einstein Field Equation of General Relativity from first principles, the constant should be there I wouldn't go that far: Rather, adding it doesn't violate any of the heuristics used to come up with the field equations or action. So to avoid bias, one should keep it around. However, in the absence of observational evidence to constrain its value, it's also justified to start an…

> adding it doesn't violate any of the heuristics used to come up with the field equations or action. So to avoid bias, one should keep it around

Yes, that's what I was trying to say. I didn't mean that including it mathematically in the equations necessarily requires one to adopt a non-zero value for it; you are quite correct that one shouldn't do that unless one has observational evidence to back it up (and cosmologists in fact didn't adopt a non-zero value until observational evidence required it).

Re: The universe is expanding faster than expected

#236

Post by gpderetta » Tue, Dec 22, 2020, 9:57 PM UTC

pdonis wrote: post #233

gpderetta wrote: post #219

Earlier quoted context omitted.

Re: accellerating expansion, I was only commenting on how the expansion itself is a consequence of GE. Thanks for your point about the constant being actually required. I do not understand the math, but is this similar on how integrals always have a constant as a free parameter that need to be determined by other means? I assume that Einstein originally set the constant to exactly balance the expansion, but later set…

> is this similar on how integrals always have a constant as a free parameter that need to be determined by other means? Not really, no. It's a consequence of the assumptions made when the Einstein Field Equation (EFE) is derived from a Lagrangian using the principle of least action. The assumptions are that the Lagrangian should be a Lorentz scalar (which is required of any Lagrangian) and that it should include no…

Thanks for the explanation in this and the sibling comment!

Re: The universe is expanding faster than expected

#237

Post by ngcc_hk » Tue, Dec 22, 2020, 10:41 PM UTC

pdonis wrote: post #124

User23 wrote: post #19

One thing that really frustrates me about cosmology, or at least popular science reporting and books, is a particular kind of lack of intellectual rigor. Specifically, even as a layman, it's clear that observation and model are being conflated. Cosmology is a science in the broadest sense of being a field of human knowledge, but it isn't a science the way that, for example, physics is. It would better be described as…

> It would better be described as a phenomenology Not really, no, because current cosmological models are not derived solely by looking at cosmological phenomena and coming up with phenomenological equations that describe them. (That is not to say that past cosmological models didn't do that; only that our current ones, roughly since the early to mid-20th century, don't.) They are derived by starting from laws of phy…

Great touch. Actually I think we have model based science quite many decades now, but those model need to be anchored on testable and refutable something. (Phenomena is loaded word as I wonder it assumes model free and equipment free a bit). Model and refutable is key here.

We still have the qm to deal with. Just hope our model is even understandable. But at least that model is refutable.

As for correctness, I suspect we might never see 100% ever though. That is partly we are just have a century of major re-think about the fossilization of good science. And there is just happen our maths and mind can understand quite a bit of the universe.

There is always something to be found is good. And there is no guarantees we will know it all. That is good too. As it keeps the most important of science and philosophy alive:

Curiosity.

Re: The universe is expanding faster than expected

#238

Post by chanakya » Wed, Dec 23, 2020, 12:20 AM UTC

nuclear_eclipse wrote: post #133

chanakya wrote: post #126

Can someone explain why the universe is expanding at all? I've heard it said "it's the momentum from the Big Bang". That sounds reasonable, but then we're told that it's not that the galaxies are flying apart, it's that the "basic metric of space itself is expanding". I can understand momentum from an explosion, but why would "space itself" expand because of an explosion?

If you can answer the why, then I think physicists around the world would really like to know. We "know" the universe is expanding because we can observe the effects (eg, red shift of further away stars/galaxies), but the "why" has been the big, fundamental question for as long as it was predicted and/or observed. It's almost certainly related to some property of physics that we don't yet understand, but ultimately,…

Thanks. So it's not related to Einstein's General Theory? In discussions around the cosmological constant, static vs. expanding universe and energy of space, it feels like the expansion is a consequence of the General Theory, and that all we need is to fill in some constants, like the amount of matter in the universe, and the actual energy of space.

Is the theory missing in some more fundamental way, like the lack of unification of gravity and quantum mechanics?

Re: The universe is expanding faster than expected

#239

Post by rytill » Wed, Dec 23, 2020, 8:43 PM UTC

xwdv wrote: post #217

rytill wrote: post #20

Earlier quoted context omitted.

I don't see how it's possible to un-make-something-happen. It already happened. It's gone. You can't kill what's already dead.

Imagine they told you that when you die, not only will you be gone, but then they will go back and undo everything you ever did and basically make it like you never existed at all. It didn’t happen, you never happened.

It happened, therefore it happened somehow, someplace. What’s the difference between something having happened and not having happened? It doesn’t intrinsically mean anything.

In addition, most humans learn to accept that there’s a good chance nothing they do will have an eternal effect on reality without needing any strange frameworks.

Re: The universe is expanding faster than expected

#240

Post by diggernet » Wed, Dec 23, 2020, 9:40 PM UTC

mathattack wrote: post #73

simonh wrote: post #61

Earlier quoted context omitted.

There are multiple explanations for why the sky isnt lit up with radio and laser signals from advanced civilisation. The great filter is one explanation, that there are existential crises or threats that wipe out most civilisations or cause them to collapse to subsistence level. Nuclear war, biological weapons, ecological collapse, paper clip maximising AI, etc. I once ran a Traveller RPG exploration campaign where o…

I hope you snuffed them out before they took over the universe. The surprisingly addictive game is below. https://www.decisionproblem.com/paperclips/

I hate you