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Webb and Hubble confirm Universe's expansion rate

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Re: Webb and Hubble confirm Universe's expansion rate

#111

Post by AlecSchueler »

pdonis wrote: post #97
arcastroe wrote: post #46

I've asked this question before, but I don't think I received a good answer, so figured I'd try asking again. How do we know that galaxies are accelerating away from us and not moving at constant speed? People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration. However, wouldn't we expect to see the same observation even without any…

> How do we know that galaxies are accelerating away from us and not moving at constant speed? More precisely, we see that galaxies started accelerating away from us a few billion years ago; before that they were decelerating (moving away from us but with the "speed" decreasing instead of increasing). > People often point to the observation that the further away a galaxy is, the faster it appears to be moving away fr…

By few billion are you talking like 3 billion? Why the change?

Re: Webb and Hubble confirm Universe's expansion rate

#112

Post by yen223 »

fsckboy wrote: post #77

given that a telescope conceived while Bill Clinton was president in the 1990's got named so as to complete the name "Webb Hubble", let's please please please call the next telescope "Chelsea". https://pagesix.com/wp-content/uploads/sites/3/2020/02/chels... https://www.wnd.com/wp-content/uploads/2015/10/Webb-Hubbell_... (Chelsea Clinton's resemblance to Clinton family friend Webb Hubble has prompted speculation...)

I now really want to know what prompt was used to generate this text

Re: Webb and Hubble confirm Universe's expansion rate

#113

Post by pdonis »

arcastroe wrote: post #94
digging wrote: post #72

Earlier quoted context omitted.

It's because the movement is ascribed to the expansion of space itself, not the individual galaxies. We don't have any reason to believe galaxies are moving in random directions at random speeds (not at scales that explain the redshifts we call the expansion of the universe). In your explanation, I think we'd expect to see some very distant, very slow-moving galaxies moving toward us. And there may be some very fast-…

> In your explanation, I think we'd expect to see some very distant, very slow-moving galaxies moving toward us Thank you, but I suppose I'm not really questioning the big bang piece. My question was mostly in regards to the continued acceleration piece. Feel free to disregard the "in random directions" part of my original post. I'm picturing more of an explosion in empty space. A firework or granade of sorts. Any in…

> I'm picturing more of an explosion in empty space.

No, that's not what the big bang is.

> this would imply a "center" to the explosion, which I've also heard is not the case

That's correct. The big bang does not work like anything ordinary that you are used to imagining. The math is straightforward and unambiguous, but there is no good ordinary language description that corresponds to the math.

Re: Webb and Hubble confirm Universe's expansion rate

#114

Post by pdonis »

willis936 wrote: post #67
runeb wrote: post #62

Earlier quoted context omitted.

Their light is more red shifted the farther away they are. I'm no expert on this, but I believe in a constant-speed scenario they would have equal red shift no matter the distance

The assumption made here is that relative velocity is the only method that would redshift light. Gravitational redshift is a thing and our model of gravity is incomplete.

> The assumption made here is that relative velocity is the only method that would redshift light.

Not in our actual model of the universe, no. The redshift of light is determined by the spacetime geometry and the worldlines of the emitter and receiver. That is a general formula that works in any spacetime.

> Gravitational redshift is a thing

Not for the universe as a whole, no. Gravitational redshift is only meaningful in certain kinds of spacetimes, namely stationary spacetimes (which, roughly speaking, describe objects that either don't change with time at all, or which are periodic, like a rotating planet or star). The spacetime that describes our universe as a whole is not stationary and there is no meaningful concept of gravitational redshift.

> our model of gravity is incomplete

In the sense that we do not have a quantum theory of gravity, yes. But that does not affect anything under discussion here. Our current theory of gravity, GR, works fine for treating the expansion of the universe and whether or not it is accelerating.

Re: Webb and Hubble confirm Universe's expansion rate

#115

Post by dmd »

lagrange77 wrote: post #109
tzs wrote: post #28

A little background based on a few articles about this plus my recollection of PBS Space Time videos on this: There are at least two ways to try to figure out the rate of expansion of the universe (which is called the Hubble Constant). • From variations in the cosmic microwave background (CMB) which are the result of certain conditions in the early universe it is possible to figure out what the expansion rate should…

Interesting, thanks! Out of curiosity: Can we actually differentiate between the expansion of space itself and the drifting of objects within it in the same direction?

You would have to come up with an explanation for such drifting to be occurring across the entire visible universe in precisely the same way, across billions and billions of light years.

Re: Webb and Hubble confirm Universe's expansion rate

#116

Post by explaininjs »

Veserv wrote: post #68
explaininjs wrote: post #26

Earlier quoted context omitted.

> Of course, all the results have error bars. Not to mention the underlying philosophical assumptions, such that the "rate of time" has been constant across all of... time. Aka: How do we know a "year" 13 Billion "years" ago bears any resemblance to one now? What would it mean for it not to?

Well for one, if the laws of physics are not time invariant (i.e. the laws of physics are not the same at all points in time) then energy can be created or destroyed [1]. So that would be quite a shocker. [1] https://en.m.wikipedia.org/wiki/Noether%27s_theorem

Do you think the current amount of energy in the universe is 0? If not, how was it created?

Re: Webb and Hubble confirm Universe's expansion rate

#117

Post by pdonis »

AnimalMuppet wrote: post #90
arcastroe wrote: post #46

I've asked this question before, but I don't think I received a good answer, so figured I'd try asking again. How do we know that galaxies are accelerating away from us and not moving at constant speed? People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration. However, wouldn't we expect to see the same observation even without any…

I'm not an expert, but I think it's like this: If the universe were expanding uniformly, we would see galaxies moving away from us. The further away they are, the faster they would move. Distance and velocity would have a linear relationship, with the Hubble Constant as the scaling factor. But what we actually see is that, if we measure precisely enough, galaxies further away are moving faster than that. The conclusi…

> galaxies further away are moving faster than that

No, you have it backwards. Accelerating expansion means, roughly speaking, that we see galaxies further away moving away slower than a "uniform" expansion would predict. Remember that we are seeing galaxies further away as they were a longer period of time ago--so "accelerating expansion" means the universe was expanding slower then, when the light was emitted, than it is now.

Actually, though, we don't observe the distance to a galaxy directly. We infer it from other observations. The actual observed quantities are redshift, brightness, and angular size, and the relationship between those three observed quantities is what tells us the expansion history of the universe.

Re: Webb and Hubble confirm Universe's expansion rate

#118

Post by pdonis »

AlecSchueler wrote: post #111
pdonis wrote: post #97

Earlier quoted context omitted.

> How do we know that galaxies are accelerating away from us and not moving at constant speed? More precisely, we see that galaxies started accelerating away from us a few billion years ago; before that they were decelerating (moving away from us but with the "speed" decreasing instead of increasing). > People often point to the observation that the further away a galaxy is, the faster it appears to be moving away fr…

By few billion are you talking like 3 billion? Why the change?

Meaning, why did the expansion change from decelerating to accelerating a few billion years ago? Because that was when the density of matter, which had dominated the dynamics until then, became smaller than the density of dark energy, which has dominated the dynamics since then. The dark energy density doe not change with time, but the density of matter decreases as the universe expands.

Re: Webb and Hubble confirm Universe's expansion rate

#119

Post by willis936 »

pdonis wrote: post #114
willis936 wrote: post #67

Earlier quoted context omitted.

The assumption made here is that relative velocity is the only method that would redshift light. Gravitational redshift is a thing and our model of gravity is incomplete.

> The assumption made here is that relative velocity is the only method that would redshift light. Not in our actual model of the universe, no. The redshift of light is determined by the spacetime geometry and the worldlines of the emitter and receiver. That is a general formula that works in any spacetime. > Gravitational redshift is a thing Not for the universe as a whole, no. Gravitational redshift is only meaning…

>Our current theory of gravity, GR, works fine for treating the expansion of the universe and whether or not it is accelerating.

Then why are there phantoms in the data that need dark matter and dark energy to make the supposed working model fit them?

Re: Webb and Hubble confirm Universe's expansion rate

#120

Post by explaininjs »

itishappy wrote: post #107
vlovich123 wrote: post #71

Earlier quoted context omitted.

Unfortunately we can’t observe fundamental things like “what are the rules of physics and time at the beginning of the universe?”. We look for clues and make large assumptions, but given that the universe experienced a 10^78 factor expansion during the Big Bang, assuming that actually happened, then why would it make sense to assume that the rules of the universe today are the rules for the very early beginning of th…

Entirely reasonable assumptions! Our models match surprisingly well though... The CMB has a blackbody spectrum that aligns with predictions, we see galaxies more or less when and where we'd expect them, stellar populations look like what we'd expect for a universe made of hot hydrogen, and more! It's not quite perfect, but modern physics explains stuff really quite well even billions of years ago!

Modern physics is guided by those observations, they can't be then used as an argument for its veracity.

Let's say I walk into a room and observe someone writing a tally mark on a chalk board once every second. I count 4x10^17 tally marks. I might assume that 4x10^17 seconds ago that same person entered the room and started tallying. I might even observe for the next 4x10^17 seconds they continue to tally. Heck I might even see a recorder going that when I play back at what I assume to be 1x speed, has chalk scratches at regular intervals for 4x10^17 seconds. I still don't have any actual evidence that they started those 8x10^17 seconds ago.

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