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Analysis indicates that the universe’s expansion is not accelerating

ras.ac.uk

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Re: Analysis indicates that the universe’s expansion is not accelerating

#171

Post by omnicognate »

mr_toad wrote: post #168
omnicognate wrote: post #91

Earlier quoted context omitted.

Time dilation means that you can get anywhere while experiencing an arbitrarily small amount of time. You can cross the galaxy in a second as far as special relativity is concerned. (With the expenditure of insanely vast amounts of energy, ofc.) To an observer back home you'd look like you're travelling at merely extremely close to the speed of light, but to you the journey would take a second.

> You can cross the galaxy in a second as far as special relativity is concerned. Sure, but the rest of the universe will keep on changing. In 90 billion years it’s going to be a very old universe. Galaxies will become consolidated and isolated, fewer young stars will be born. Only the dim light of red dwarf stars will shine among a graveyard of dead stars.

Hey, I didn't say it would be fun :-D

Tau-Zero by Poul Anderson explores this, BTW. It's a great little sci fi novel about a spaceship doomed to accelerate at 1G indefinitely.

It is significant from a colonisation PoV. With sufficient acceleration capability and the ability to survive travelling through the interstellar medium at extreme velocity (rather than getting vaporised by a mote of dust), a single generation of humans could in theory colonise the whole galaxy within their own lifespans. Indeed, some of them could even come back together and meet again after visiting those distant worlds, on an Earth many millions of years older, if their worldlines end up with similar proper times.

Re: Analysis indicates that the universe’s expansion is not accelerating

#172

Post by wtcactus »

ghtbircshotbe wrote: post #164
jimbo808 wrote: post #15

Anyone know how credible this is? If true, then that means the big bounce is back on the menu, and the universe could actually be an infinitely oscillating system.

How does an infinitely oscillating universe comply with the 2nd law of thermodynamics?

It doesn’t. Either the 2nd law is incomplete or the idea of bouncing universes starting from scratch with a clear state in entropy is impossible:

https://arxiv.org/abs/1010.5513

Re: Analysis indicates that the universe’s expansion is not accelerating

#173

Post by wtcactus »

jumploops wrote: post #139
jimbo808 wrote: post #15

Anyone know how credible this is? If true, then that means the big bounce is back on the menu, and the universe could actually be an infinitely oscillating system.

“We can’t observe the whole universe, so cosmology is not really about the universe. It’s about the observable patch and the assumptions we make about the rest.” (paraphrasing George Ellis) We’re in a bounding sphere, with a radius that’s roughly 46.5 billion lightyears, so any observation we make may be true for our local observable range, but there’s no (known) way to know what’s beyond that sphere.

Right, but everywhere we look the universe is roughly the same at same distances, so it’s acceptable to extrapolate.

Re: Analysis indicates that the universe’s expansion is not accelerating

#174

Post by wtcactus »

cosmicjoe wrote: post #132

As a non-scientist I've always found the Cosmic Distance Ladder as likely to be inaccurate due its assumption about the constant brightness of Standard Candle stars over their lifetime, and the compounding of error at each rung of the ladder. Direct measurement of the CMB seems to be simpler with less chance of error. https://en.wikipedia.org/wiki/Cosmic_distance_ladder

The supernovas type Ia luminosity depends on their composition and that takes into account both the age of the supernova and of the donator star. And that can be inferred by the luminosity curve of the supernova.

https://en.wikipedia.org/wiki/Type_Ia_supernova

Re: Analysis indicates that the universe’s expansion is not accelerating

#175

Post by Razengan »

recursivecaveat wrote: post #125
Razengan wrote: post #30

Earlier quoted context omitted.

So wait, individual stars aren't getting further apart? Galaxies aren't getting "bigger"/more diffuse?

Imagine the universe as a giant balloon. Inside are little miniature balloon stars floating around, tied with string into balloon galaxies. If we heat the air: the big balloon expands, the clusters of mini-balloons spread out from the other clusters, but the clusters don't get any more diffuse. The string is way way too strong to be overpowered by the separating force from the expansion of the gas over short distance…

I mean, this is tricky to even ask: is there still expansion INSIDE galaxies, BUT it's countered by gravity?

Or is there no expansion within galaxies at all?

i.e. is dark energy or whatever that causes expansion only present in the absence of matter, or is it present everywhere regardless of matter, but because matter also has its own gravity the expansion is not visible/relevant?

Re: Analysis indicates that the universe’s expansion is not accelerating

#176

Post by aatd86 »

aatd86 wrote: post #160
pinkmuffinere wrote: post #75

Earlier quoted context omitted.

You’re being downvoted, but your point is true — something can exist “by definition”, and yet not exist in our real world. The thing that exists “by definition” is just a version that we have imagined to exist by definition. But imagining something with property X doesn’t imply anything can actually be found with property X. Side-note: the deontological argument is an argument for the existence of God, which uses the…

Nope :) It 's not about that. It's not because I imagine that there is a banana in front of me that there will be. It's not tied to material existence in that way. It's perhaps another notion of existence which should be more mathematical. You could think it as "God" provably existing as an idea but that might or might not be realized probabilistically, in our material world. The idea exists obviously. Same as "Zeus"…

Funny thing is to ask: 'Is blue, blue?' Now with existence: 'Does existence, exist?' And then a bit differently: 'Is nothing something?'

We see that these are different types of impredicativity.

Existence just needs itself to define itself. Nothing cannot exist if nothing actually somehow is. It needs existence. Blue is a word. It does not exhibit the characteristic is describes. The set of all things blue does not contain the proposition 'blue'.

While the set of all things that exist contains itself? Sweet baby Ouroboros ;D

Re: Analysis indicates that the universe’s expansion is not accelerating

#177

Post by mr_toad »

jfengel wrote: post #39
candiddevmike wrote: post #6

Aside from unanswerable questions (has the universe started to fill it's container? Is a simulation property nearing "1"?), does this make long distance space travel feasible again? I thought there was something around the universe is expanding too fast to visit places like Alpha Centuri (and preventing visitors to us).

The limit to space travel is the Rocket Equation, which says that you require exponential fuel to reach higher speeds. Alpha Centauri isn't going anywhere, but it will take millennia of travel even with wildly optimistic assumptions. Also note that there isn't any "container" to fill up. It could well be infinite. It's just that we will be forever limited to a finite subset, even in theory.

There are theoretical designs using antimatter (pion rockets, or beam core engines) that could reach 0.4c - 0.7c, which puts Alpha Centauri at decades away, not millennia.

https://arxiv.org/pdf/1205.2281 https://ntrs.nasa.gov/api/citations/20200001904/downloads/20...

Re: Analysis indicates that the universe’s expansion is not accelerating

#178

Post by naasking »

cosmicjoe wrote: post #132

As a non-scientist I've always found the Cosmic Distance Ladder as likely to be inaccurate due its assumption about the constant brightness of Standard Candle stars over their lifetime, and the compounding of error at each rung of the ladder. Direct measurement of the CMB seems to be simpler with less chance of error. https://en.wikipedia.org/wiki/Cosmic_distance_ladder

> I've always found the Cosmic Distance Ladder as likely to be inaccurate due its assumption about the constant brightness of Standard Candle stars over their lifetime

Stars are just basic nuclear physics and gravity, that's why they're expected to be stable and predictable.

> Direct measurement of the CMB seems to be simpler with less chance of error.

Direct measurement of the CMB doesn't tell you anything on its own, you have to interpret the data in terms of a model. If you have a completely different model, say one without dark energy or without dark matter, CMB measurements would tell you something different than LCDM.

Re: Analysis indicates that the universe’s expansion is not accelerating

#179

Post by the__alchemist »

JumpCrisscross wrote: post #84

“Supernova (SN) cosmology is based on the key assumption that the luminosity standardization process of Type Ia SNe remains invariant with progenitor age. However, direct and extensive age measurements of SN host galaxies reveal a significant (5.5σ) correlation between standardized SN magnitude and progenitor age, which is expected to introduce a serious systematic bias with redshift in SN cosmology. This systematic…

I did a deep dive into cosmology simulations ~a year ago. It was striking how much is extrapolated from the brightness of small numbers of galaxy-surface pixels. I was looking at this for galaxies and stars, and observed something similar. The cosmology models are doing their best with sparse info, but to me it seemed that the predictions about things like Dark Matter and Dark Energy are presented in a way that's too confident for the underlying data. Not enough effort is spent trying to come up with new models. (Not to mention trying to shut down alternatives to Lambda CDM, or a better understanding of the consequences of GR, and the assumptions behind applying Newtonian instant-effect gravity in simulations).

Whenever I read things like "This model can't explain the bullet cluster, or X rotation curve, so it's probably wrong" my internal response is "Your underlying data sources are too fuzzy to make your model the baseline!"

I think the most established models are doing their best with the data they have, but there is so much room for new areas of exploration based on questioning assumptions about the feeble measurements we can make from this pale blue dot.

Re: Analysis indicates that the universe’s expansion is not accelerating

#180

Post by cvoss »

basch wrote: post #167
nabakin wrote: post #32

> type Ia supernovae, long regarded as the universe’s "standard candles", are in fact strongly affected by the age of their progenitor stars. A key point in the article. From what I understand, this is the main way we measure things of vast distance and, from that, determine the universe's rate of expansion. If our understanding of these supernovae is wrong, as this paper claims, that would be a massive scientific br…

This is mostly my physics ignorance talking, but if we measure distance in space-time and not just space, and speed or velocity is space-time/time (which somehow are both relative to each other) and the derivative of velocity is acceleration, cant acceleration mean either expanding "faster" in the sense of distance OR time speeding up or slowing down? All of it seems so self referential its hard to wrap around.

We measure distance in space, and time intervals in time, and so velocity is just plain old distance/time. Special relativity doesn't change that. What changes is that if you start traveling at a different velocity, your measurements of distances and time intervals deviate.

The expansion rate of the universe is not a velocity in the usual sense of distance/time. It's actually in units of velocity/distance, which reduces to 1/time. An expansion rate of r Hertz means that a given span of distance intrinsically doubles every 1/r seconds. The objects occupying the space don't "move" in any real sense due to expansion. They just wind up farther apart because space itself grew.

And, just like measurements of distance and time, measurements of the expansion rate change if you change your velocity. There is a special velocity in our universe which causes the expansion in all directions to be the same. From this special perspective, which is traveling at a kind of cosmic "rest" velocity, you can calculate the expansion rate. It turns out that the Sun is traveling at approximately 370 km/s with respect to that special "rest" velocity.

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