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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

#251

Post by munksbeer »

baq wrote: post #233
munksbeer wrote: post #220

Earlier quoted context omitted.

What does "space is expanding" mean? That the distance between objects is increasing? How can you tell the difference between "space expanding" and "objects moving in a non-expanding space"? Is there any way to tell the difference or is it just that all objects are moving away at the exact speeds that satisfies the "space expanding" explanation and nothing else? But then, I'm back to what does "space is expanding" me…

space expanding means what it literally says: there is more space everywhere at once. there was less a moment ago and now there's more. the longer the distance, the more space gets added in between, thus the effect is extreme on universe scale and undetectable on planetary scales.

Well yes, but did you read my post in detail? I asked what the difference was between "space expanding" vs "objects moving away from each other".

And secondly, what is the mechanism for "space expanding"? What is "space" in this context? What actually is expanding?

Re: Webb and Hubble confirm Universe's expansion rate

#252

Post by mr_toad »

Kerbonut wrote: post #155
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…

Could it be the rate of expansion is not constant?

https://en.wikipedia.org/wiki/Quintessence_(physics)

Re: Webb and Hubble confirm Universe's expansion rate

#253

Post by joshjje »

physicles wrote: post #169

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 prima…

I imagine there must be lots of gravity lensing going on as well, not sure how they deal with that.

Re: Webb and Hubble confirm Universe's expansion rate

#254

Post by mr_mitm »

pfannkuchen wrote: post #210

I don’t really understand how we can make conclusions about the entire universe when literally all of the data is collecting from one point in space. Couldn’t there be local effects that obfuscate global behavior? Is there an implicit caveat asterisk on all such statements that is like “as far as we can possibly tell from the data we have” and the reality is we really, really don’t know for sure?

> “as far as we can possibly tell from the data we have”

Isn't that self evident in all cases?

In cosmology, the implicit assumptions are the Corpernican principle and that GR is correct. That is covered in introductory texts, but unless stated otherwise, it is usually not mentioned, especially not in press releases.

I mean, maybe you're just a Boltzmann brain floating in space and nothing is real. Fun to think about, sure, but expecting to state the basics in any and all cases is unrealistic.

Re: Webb and Hubble confirm Universe's expansion rate

#255

Post by mr_toad »

explaininjs wrote: post #26
orra wrote: post #15

Earlier quoted context omitted.

> we’ve been using the 13.8 billion cosmological age estimate ever since I started brushing up my layman's understanding of the subject. I remembered the age of the universe as as 13.7 billion years, but I wasn't sure why that was. Well, the initial WMAP results in 2003 supported an age of 13.7 billion years. Later results nudged this upwards to 13.8 billion years. Of course, all the results have error bars.

> 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?

Rates are measured with time as the divisor. What does rate of time even mean?

Re: Webb and Hubble confirm Universe's expansion rate

#256

Post by mr_toad »

dartos wrote: post #34
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?

I’d assume that we have some notion of how the laws of physics have changed, if at all, since the Big Bang. We measure time in vibrations of a cesium isotope IIRC

“Before” the Big Bang it’s not even clear that there were laws of physics.

Re: Webb and Hubble confirm Universe's expansion rate

#257

Post by pigpang »

munksbeer wrote: post #251
baq wrote: post #233

Earlier quoted context omitted.

space expanding means what it literally says: there is more space everywhere at once. there was less a moment ago and now there's more. the longer the distance, the more space gets added in between, thus the effect is extreme on universe scale and undetectable on planetary scales.

Well yes, but did you read my post in detail? I asked what the difference was between "space expanding" vs "objects moving away from each other". And secondly, what is the mechanism for "space expanding"? What is "space" in this context? What actually is expanding?

Spacetime is 4D array of points. It's like a movie file, but with 3D frames instead of 2D frames (pictures). Expansion of space means that coordinates of points in a frame changed to move away from us, to match movie (model) with reality.

Re: Webb and Hubble confirm Universe's expansion rate

#258

Post by mr_toad »

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…

> It's impossible to triangulate those huge distances

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

Re: Webb and Hubble confirm Universe's expansion rate

#259

Post by mr_toad »

javajosh wrote: post #153
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…

>It's impossible to triangulate those huge distances Galaxies are BIG. Andromeda is faint, but the same angular size as the moon. It's 2.5Mly away, but it's also 150kly across. Over a long enough time line you could do triangulation on it. In fact it's moving toward our galaxy, but very, very slowly compared with its diameter at 110kps. But yeah, in theory you could do triangulation on it over a very long period of t…

For triangulation to work you need to move, not your target. Triangulation is only used for objects within about 1000 parsecs, where we can triangulate using the movement of the Earth along its orbit.

Re: Webb and Hubble confirm Universe's expansion rate

#260

Post by isolli »

isolli wrote: post #225

[Naive question warning] What if the cosmological constant (from which, if I understand correctly, the Hubble constant is derived) is not constant? Could a changing comological "constant" explain the discrepancy between the various methods of calculating the Hubble constant? From Wikipedia [0]: The cosmological constant "was revived and reinterpreted as the energy density of space, or vacuum energy, that arises in qu…

Thank you to both commenters, who pointed me to this theory: https://en.wikipedia.org/wiki/Quintessence_(physics)
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