What does expand: Space or Spacetime?

[Smattering]

Hello everybody,

When we say that the universe expands, what exactly do we refer to: Space or space time? Is it only a spatial expansion or also a temporal expansion?

If it is also a temporal expansion , what are the implications w.r.t. to inflation and accelerating expansion?Robert

 

[Chalnoth]

It refers to space expanding. Specifically, the distance between far-away objects gets greater over time. The time coordinate chosen does not expand.

 

[HunterThomson]

It concerns only space. It expands faster than it was before and now we have to find the reason why. Currently, we believe the inflation occurs because of the dark energy, but it remains to be a mystery .

 

[phinds]

Saying that space expands is not exactly incorrect, but it is misleading. It implies that space is a "fabric" or something that can be stretched. Better to just say that things get farther apart from each other since there is no "thing" there that actually expands. This can seem like just semantics but it really isn't. Google "metric expansion".

 

[Smattering]

It refers to space expanding. Specifically, the distance between far-away objects gets greater over time. The time coordinate chosen does not expand.

This is what I always thought, but then I saw a feature with a German astrophysicist where he argued that in the big bang, not only space but rather space time expanded.

The statement appeared a bit strange to me, because expansion clearly seems to be a temporal process. You compare some measure at different points in time, and then you find out that the "later" values are larger. That's how I interpret expansion.

Saying that space expands is not exactly incorrect, but it is misleading. It implies that space is a "fabric" or something that can be stretched. Better to just say that things get farther apart from each other since there is no "thing" there that actually expands. This can seem like just semantics but it really isn't. Google "metric expansion".

Thanks for the clarification, but that was always clear for me. I do not think that expansion implies stretching

 

[Chalnoth]

This is what I always thought, but then I saw a feature with a German astrophysicist where he argued that in the big bang, not only space but rather space time expanded.

Saying that space-time expanded really isn't accurate. When you look at the whole space-time structure, the expansion appears as space-time curvature.

The statement appeared a bit strange to me, because expansion clearly seems to be a temporal process. You compare some measure at different points in time, and then you find out that the "later" values are larger. That's how I interpret expansion.

I think the point is that if you have a clock at different points in time in the universe, that clock behaves the same according to the coordinate time we usually use to describe the expansion (that is, one second in the coordinate time is one second on the clock, as long as that clock is stationary with respect to the expansion).

 

[HunterThomson]

Currently, we believe the inflation occurs because of the dark energy

https://writingservice.essayhave.com/data/uploads/dark-energy.jpg

 

[Smattering]

@HunterThomson: So did the amount of dark energy increase?

 

[phinds]

@HunterThomson: So did the amount of dark energy increase?

The amount of dark energy is always increasing. That's one of the mysteries of dark energy. It has a constant density per unit volume so when things get farther apart, there is more dark energy between them.

 

[Chronos]

Expansion is most easily explained by the simplest version of dark energy - a cosmological constant. If the dark energy density of the universe is truly a constant then it constitutes the simplest model to explain expansion. This is why efforts to measure dark energy density during different epochs in the universe are a hotbed of research activity. If it turns out to vary over time, it will deeply impact cosmology. The results to date remain inconclusive, but, do not rule out a cosmological constant. The tricky part is DE, not unlike dark matter, is not something that lends itself to direct measurement . We are constrained to measure it by examining the motions of matter in the distant universe. It was suspected a few years ago that supernova data accumulated since the introduction of DE into our cosmological model suggested a value for w of less than -1, which would disfavor the cosmological constant version of DE. Last I heard on this front is uncertainty in the data was too large to permit any firm conclusions. For further discussion on why the cosmological constant is a simple, yet natural explanation for expansion, see http://arxiv.org/abs/1002.3966, Why all these prejudices against a constant?.

 

[Smattering]

That's one of the mysteries of dark energy. It has a constant density per unit volume so when things get farther apart, there is more dark energy between them.

So, the tendency to expand can be considered an intrinsic property of space, as long as there are no other interference factors that compensate this tendency to expand?

 

[phinds]

So, the tendency to expand can be considered an intrinsic property of space, as long as there are no other interference factors that compensate this tendency to expand?

No, I don't think it is "an intrinsic property of space", but since we dont' know WHAT it is, no one can say for sure. If you are asking whether or not it is the way things work in our universe, then obviously yes.

 

[Smattering]

If you are asking whether or not it is the way things work in our universe, then obviously yes.

I was just wondering about the simplest way to describe the existing observations. And if the density per unit volume stays constant, although space continuously expands, then it seems to me that the simplest description might actually be to treat it as an intrinsic property of space.

 

[martinbn]

Expansion is most easily explained by the simplest version of dark energy - a cosmological constant.

You mean accelerated expansion. For expansion you don't need dark energy.

 

[cristo]

Currently, we believe the inflation occurs because of the dark energy

This isn't correct. Technically, inflation refers to the accelerated expansion in the very early universe (right after the big bang); dark energy is responsible for the accelerated expansion that the universe is perceived to be undergoing in the present epoch.

The amount of dark energy is always increasing. That's one of the mysteries of dark energy. It has a constant density per unit volume so when things get farther apart, there is more dark energy between them.

Your first sentence also isn't technically correct: the "amount" of dark energy is not increasing, it's energy density is constant. The different is that all other energy/matter species decay with the expansion, so the relative contribution of dark energy to the energy budget is increasing.

 

[cristo]

I was just wondering about the simplest way to describe the existing observations. And if the density per unit volume stays constant, although space continuously expands, then it seems to me that the simplest description might actually be to treat it as an intrinsic property of space.

I think you're mixing up two concepts here - expansion and accelerated expansion. The former is just really an initial condition, while the latter requires some new explanation. This could either be some property of spacetime (such as a cosmological constant; but that itself brings other issues), or some new, weird matter species.

 

[phinds]

This isn't correct. Technically, inflation refers to the accelerated expansion in the very early universe (right after the big bang); dark energy is responsible for the accelerated expansion that the universe is perceived to be undergoing in the present epoch.
Your first sentence also isn't technically correct: the "amount" of dark energy is not increasing, it's energy density is constant. The different is that all other energy/matter species decay with the expansion, so the relative contribution of dark energy to the energy budget is increasing.

Yeah, I guess my wording was sloppy. The amount of dark energy between us and a galaxy 3 billion light years away, in say a conceptual tube the diameter of the Earth, is increasing with time BECAUSE its density is constant but the distance increases. It's a messy concept and I'm never quite sure how to talk about it.

 

[timmdeeg]

I was just wondering about the simplest way to describe the existing observations. And if the density per unit volume stays constant, although space continuously expands, then it seems to me that the simplest description might actually be to treat it as an intrinsic property of space.

Being a vacuum energy the dark energy or the cosmological constant resp. might be an intrinsic property of the vacuum, rather than of space. From this point of view, what happens to the vacuum energy if the universe expands? Does it get "diluted"? I don't think so, whereas the density of matter definitively does. Of course, as we don't know the nature of the dark energy, we can't exclude a still unknown time dependence of its density.

 

[Chalnoth]

Being a vacuum energy the dark energy or the cosmological constant resp. might be an intrinsic property of the vacuum, rather than of space. From this point of view, what happens to the vacuum energy if the universe expands? Does it get "diluted"? I don't think so, whereas the density of matter definitively does. Of course, as we don't know the nature of the dark energy, we can't exclude a still unknown time dependence of its density.

The vacuum doesn't dilute. I.e., its properties remain the same regardless of the expansion.

 

[timmdeeg]

The vacuum doesn't dilute. I.e., its properties remain the same regardless of the expansion.

So, in case the vacuum energy is such a property, it remains the same. If I understood you correctly.

 

[Chalnoth]

So, in case the vacuum energy is such a property, it remains the same. If I understood you correctly.

Yes. And every observer will measure the same vacuum energy per unit volume.

 

[Buzz Bloom]

Hi @HunterThomson:

I found something odd in the chart you included in your post #7. Regarding Dark Matter it says:

Dark Matter also emerges in the first second.​

This makes no sense to me because dark matter is matter, and it can't be created from nothing. The baryonic matter was created from photons, but since dark matter doesn't interact with photons or baryonic matter, then it doesn't seem plausible that it can be created from photons. The only explanation that makes sense to me is the the dark matter was created simultaneously with the photons.

Where did this chart come from?

Regards,
Buzz

 

[Chalnoth]

Hi @HunterThomson:

I found something odd in the chart you included in your post #7. Regarding Dark Matter it says:

Dark Matter also emerges in the first second.​

This makes no sense to me because dark matter is matter, and it can't be created from nothing. The baryonic matter was created from photons, but since dark matter doesn't interact with photons or baryonic matter, then it doesn't seem plausible that it can be created from photons. The only explanation that makes sense to me is the the dark matter was created simultaneously with the photons.

Where did this chart come from?

Regards,
Buzz

At very high temperatures that would have existed in the very early universe, it is expected that dark matter interacted quite readily with normal matter. It wouldn't have interacted with photons, but it's possible, for instance, that dark matter interacts with the weak nuclear force like neutrinos do. When temperatures were higher than about 1MeV (around 10 billion degrees), neutrinos interacted quite readily with normal matter. Dark matter may have done something similar.

 

[rootone]

... Dark matter may have done something similar.

Dark photons , hmm.

 

[Buzz Bloom]

When temperatures were higher than about 1MeV (around 10 billion degrees), neutrinos interacted quite readily with normal matter. Dark matter may have done something similar.

Hi @Chalnoth:

Thanks for your post.

As I try to educate myself about physics, I am often confused about the degree of scientific certainty or level of confidence regarding various pronouncements. I am guessing that what I quoted above is based on what is currently untestable theory, in contrast with such near certainty facts such as the value of physical constants. Can you clarify what the theoretical foundations are for "higher than about 1 MeV" (1) "neutrinos interact quite readily", and (2) "Dark matter may have done something similar." Are these ideas generally accepted as likely to be "true" or are they only plausible possibilities that may be somewhat controversial?

Regards,
Buzz

 

[eltodesukane]

According to general relativity, time and space form spacetime.
Spacetime is a fixed, frozen, unchanging, immutable 4-dimensional block.
It includes all of space and all of time, all past and all future.
It is not expanding, it is not moving, it is not changing. It does not sit in time, time is within it.
---
"...the past, present and future are equally ‘real’, i.e., they co-exist on equal footing.
There is no dynamics within space-time itself: nothing ever moves therein; nothing happens; nothing changes. In particular, one does not think of particles as moving through space-time, or as following along their world-lines. Rather, particles are just in space-time, once and for all, and the world-line represents, all at once, the complete life history of the particle." by Mark Stuckey
https://www.physicsforums.com/insig...ions-part-1-time-dilation-length-contraction/

 

[RMM]

Hi @HunterThomson:

I found something odd in the chart you included in your post #7. Regarding Dark Matter it says:

Dark Matter also emerges in the first second.​

This makes no sense to me because dark matter is matter, and it can't be created from nothing. The baryonic matter was created from photons, but since dark matter doesn't interact with photons or baryonic matter, then it doesn't seem plausible that it can be created from photons. The only explanation that makes sense to me is the the dark matter was created simultaneously with the photons.

Where did this chart come from?

Regards,
Buzz

I found: http://maya-gaia.angelfire.com/dark_energy_inflation.jpg

 

[phinds]

According to general relativity, time and space form spacetime.
Spacetime is a fixed, frozen, unchanging, immutable 4-dimensional block.

Not true. The "block universe" (which is what the article you linked to is talking about) is an interpretation and is not required by GR. Like all interpretations, it has its adherents and its detractors.

 

[Chalnoth]

Hi @Chalnoth:

Thanks for your post.

As I try to educate myself about physics, I am often confused about the degree of scientific certainty or level of confidence regarding various pronouncements. I am guessing that what I quoted above is based on what is currently untestable theory, in contrast with such near certainty facts such as the value of physical constants. Can you clarify what the theoretical foundations are for "higher than about 1 MeV" (1) "neutrinos interact quite readily", and (2) "Dark matter may have done something similar." Are these ideas generally accepted as likely to be "true" or are they only plausible possibilities that may be somewhat controversial?

These energies are pretty well-tested in particle accelerators, which now reach energies a million times higher than 1MeV. So yes, the statement that neutrinos interact rapidly at temperatures greater than about 1MeV is experimentally quite well-tested.

It is still unknown what particle makes up dark matter, or how it interacts with normal matter. We do know that a massive particle on the order of tens to hundreds of times the mass of a proton that interacts with the weak force could fit the currently-available evidence.

 

[Chalnoth]

According to general relativity, time and space form spacetime.
Spacetime is a fixed, frozen, unchanging, immutable 4-dimensional block.
It includes all of space and all of time, all past and all future.
It is not expanding, it is not moving, it is not changing. It does not sit in time, time is within it.

A better way of saying this is that there is no "super-time". Talking about how a 4-dimensional space-time manifold changes doesn't make sense except to say how parts of the 4-dimensional manifold differ from one another.

 

[Buzz Bloom]

We do know that a massive particle on the order of tens to hundreds of times the mass of a proton that interacts with the weak force could fit the currently-available evidence.

Hi @Chalnoth:

Thanks for your post.

I would like to understand better these ideas about DM. Can you recommend a reference?

Regards,
Buzz

 

[George Jones]

I would like to understand better these ideas about DM. Can you recommend a reference?

Try 3.3.2 "Dark Matter Relics" from Baumann's excellent lecture notes

http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf

 

[Buzz Bloom]

Hi George:

Thanks very much for the link. The lectures look quite interesting, and I expect they will entertain/educate me for quite a while.

Regards,
Buzz

 

[w4k4b4lool4]

recall however that the distinction between space and time is coordinate-dependent.

that is, you can always pick a time coordinate that does not ``expand'' (with respect to your `proper time'), or you can use conformal coordinates where both space and time components of the metric ``expand'' by the same factor.

 

[N123]

I am confused by this whole thread. I have only a superficial knowledge, so probably nothing fundamental.
1. how do you know things are moving away from each other when there is no constant scale? (Any physical scale itself would expand, would it not?)
2. If it is red-shift, how do you know that photons don't gradually expand in wavelength (like a slow decay from blue to red)?
3. What makes time so great that space always has to change (expand) but time does not have to? Is this related to the fact that we can traverse space at will in any direction but time only in forward direction? (By we I mean matter with a positive rest mass.)