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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

#161

Post by 1d22a »

pdonis wrote: post #118
AlecSchueler wrote: post #111

Earlier quoted context omitted.

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.

How is is that dark energy density does not change with time? Surely the total amount of dark energy has to be constant (energy can't be created or destroyed, and all that), and then as the universe expands, that's then the same amount of energy over a larger volume, right?

Re: Webb and Hubble confirm Universe's expansion rate

#162

Post by garrettgarcia »

narag wrote: post #105
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? There's a more basic question: How do we know the galaxies are moving? It seems (I haven't seen any other response, like... ever) that we have one and only one way to measure the speed of galaxies: the red shift. It's impossible to triangulate those huge distances and the time scale would also be a barrier, so no way of confi…

The American astronomer, Halton Arp, had a theory he called "intrinsic redshift". My limited understanding is that he saw evidence of "very close" and "very far away" structures that are connected to each other in space, which makes no sense. He theorized that redshift may also be indicative of the age of a galaxy, rather than only indicating velocity.

The interpretation of redshift as velocity is also the primary reason cosmologists think the universe is expanding.

Re: Webb and Hubble confirm Universe's expansion rate

#163

Post by __turbobrew__ »

narag wrote: post #105
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? There's a more basic question: How do we know the galaxies are moving? It seems (I haven't seen any other response, like... ever) that we have one and only one way to measure the speed of galaxies: the red shift. It's impossible to triangulate those huge distances and the time scale would also be a barrier, so no way of confi…

Degrading light sounds like an interesting idea to me, you could call it the “cosmological damper” if you will. Thought experiment: imagine you have light particles in a perfectly circular orbit around a black hole. Does the light ever fall into the black hole or does it orbit for eternity?

Re: Webb and Hubble confirm Universe's expansion rate

#164

Post by mjfl »

puzzledobserver wrote: post #149
digging wrote: post #127

Earlier quoted context omitted.

Without being able to harvest dark energy, it's actually the exact opposite.

What would a hypothetical dark energy engine look like?

two black holes in highly elliptical orbit, so they they nearly collide when they come together. at the center of mass is a cloud of iron atoms. When the black holes nearly collide tidal forces across iron nuclei rip them apart. These ripped apart atoms can then be re-harvested and used as fuel in a fusion engine. The energy loss from ripping the atoms apart is then recovered by the black holes by taking advantage of dark energy when they go apart, which increases the distance between them, increasing the gravitational potential energy.

Re: Webb and Hubble confirm Universe's expansion rate

#165

Post by at_a_remove »

narag wrote: post #105
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? There's a more basic question: How do we know the galaxies are moving? It seems (I haven't seen any other response, like... ever) that we have one and only one way to measure the speed of galaxies: the red shift. It's impossible to triangulate those huge distances and the time scale would also be a barrier, so no way of confi…

You're not going to get a simple answer because the answer is quite complex.

Astronomy is the paleontology of photons. You should take an astronomy course if you really want to know, but essentially a "ladder" was built of distances, starting with the very near and slowly building outward using various techniques and discoveries of physics as they became available. This is called the cosmic distance ladder. You start with stellar parallax, then after that you go farther with "standard candles" (particular types of variable stars). But then you have to get even further out, where you can no longer see an individual star, and then you rely on specific breeds of supernovae. Only then do you get to redshift, and compounding tons of data from step three seems to verify the redshift estimates. By the time you get to the Hubble constant, it was a huge rift between two communities over what was still a factor of two difference.

It's quite fascinating, but I can't really dump out an entire book into a comment.

Re: Webb and Hubble confirm Universe's expansion rate

#166

Post by taneq »

theoreticalmal wrote: post #141
throwawaymaths wrote: post #85

Earlier quoted context omitted.

There's a 'fringe' theory that the CMB is not the echo of the big bang but rather the redshifted black body radiation of intergalactic dust (apparently the numbers are about right). Although the primary proponent of this theory is trying to justify some sort of cyclic universe model with it, if my understanding is correct it would still be compatible with a 'standard' big bang that has the "longer" (aka galactic expa…

I can’t believe it. First they take Pluto from us, next they’re going to tell me the static on my tv set isn’t the remnants of the Big Bang, but just hot dust? My childhood is crumbling!

The hot dust is also remnants of the Big Bang, if that helps. Of course, so are we. :)

Re: Webb and Hubble confirm Universe's expansion rate

#167

Post by antod »

mjfl wrote: post #156
antod wrote: post #154

Earlier quoted context omitted.

I'm no physicist, but isn't the gravitational potential energy between two objects inversely proportional to the distance between them?

it is, in the negative direction though. So increasing the distance increases the potential energy by making -1/r closer to zero.

Heh, I had to go off and read some stuff to get my head around what "negative" potential energy would even mean (high school physics was decades ago).

So... the negative is just a convention to represent work done against the "field", and positive is work done by the field? ie more of a vector than actually being negative energy? I think I get that bit now.

So now I'm wondering if that still applies to an expanding universe vs eg a rocket leaving earth. If things aren't moving further apart by work (ie force x distance) being done against their gravitational fields so much as space time expanding, is there an increase in potential energy? And then if objects are moving apart faster than escape velocity, could that still be seen as increasing potential energy?

I think I'm confusing myself further...

Re: Webb and Hubble confirm Universe's expansion rate

#168

Post by Sprocklem »

k7sune wrote: post #132
narag wrote: post #105

Earlier quoted context omitted.

How do we know that galaxies are accelerating away from us and not moving at constant speed? There's a more basic question: How do we know the galaxies are moving? It seems (I haven't seen any other response, like... ever) that we have one and only one way to measure the speed of galaxies: the red shift. It's impossible to triangulate those huge distances and the time scale would also be a barrier, so no way of confi…

“Degrading” sounds very intuitive to me. Can the frequency of the light waves simply slow down over a very long distance/time? Or maybe the speed of light simply slows down over an unimaginably long distance? We don’t have any model to describe such behavior, but everyday objects around us all slow down one way or another, what makes light so different?

IIRC, this was one of the explanations proposed when the existence of a red shift was first noted: that the light is somehow slowly losing its energy over very long distances, becoming “redder” as it did so. It ultimately lost out to the dark energy / space-time expansion theory, although I do not recall why. Presumably there was some observation that precluded “degrading” light from being the sole explanation.

Re: Webb and Hubble confirm Universe's expansion rate

#169

Post by physicles »

The article mentions the cosmic distance ladder, which is one of my favorite things in all of science. How do we know how far away the really far stuff is? It's non-trivial and I find the history fascinating.

It all started with knowing the distance from the earth to the sun. Nobody had a clue until Richer and Cassini got within 10% in 1672. Then we nailed it down in 1769 with James Cook's voyage to Tahiti, the primary purpose of which was to observe the transit of Venus from the other side of the world.

From there if you know basic geometry, you can observe the nearby stars shift a bit when the earth goes around the sun (parallax), but that only works to about 10k light years.

Then, we discovered a couple unbelievably convenient astrophysics hacks: Cepheid variables (Henrietta Swan Leavitt, 1908) and Type 1A supernovae (Subrahmanyan Chandrasekhar, 1935, the namesake of the Chandra X-Ray Observatory). These allowed us to move out a couple more rungs on the ladder.

From there, the relationship between redshift and distance becomes significant and that takes us to the edge.

https://www.uwa.edu.au/science/-/media/Faculties/Science/Doc...

Re: Webb and Hubble confirm Universe's expansion rate

#170

Post by explaininjs »

fooker wrote: post #32
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?

Philosophical indeed, as it's impossible to define the idea of a rate of time, when the idea of rate is defined in terms on time itself.

Yes, an external reference clock is needed.

If a computer chugs along doing { counter++; } at 1 clock cycle per clock cycle for 13.7 billion clock cycles, it will think 13.7 billion clock cycles have passed when counter is 13.7 billion.

On the other hand, if a computer chugs along at one clock cycle per clock cycle for 1 clock cycle, and reads &counter and sees 13.7 billion, it will think 13.7 billion clock cycles have passed.

Either way it's perfectly capable of introspecting it's source code and logically stepping back until the memory location was 0 to see how many clock cycles would have been required to reach it's current state, but that sort of reasoning is completely devoid of meaning without both perfect knowledge of what the true start state was, and a guarantee that no external influences have occurred.

Here in reality, we know neither the our start state nor our isolation level, but the hubris of many is too string to not at least try finding some logical step-back functions and iterating them until they don't know how to go any further, then proudly proclaiming that "The Start". (after all - how could it not be The Start, look, I can iterate the inverse of my step-backward function from then to now and it matches! QED!)

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