Singularity/Expanding Universe questions

[ynot]

1. How can any size be attributed to the Singularity? Given it represents all of existence, what else is it being measured against? Can it somehow be measured against itself internally?
   
2. Is the Universe that expanded from the Singularity still a Singularity? The internal structure may have changed, but isn’t it still a single entity that represents all of existence?
   
3. Do we know the size of the Universe now (if so, how), and how do we know that it’s not infinite?
   
4. If we don’t know how big the Singularity was, or how big the Universe is, how do we know that the Universe expanded from the Singularity? Couldn’t it always have been the same size and it has just changed internally?
   
5. How does proving that the observable universe is expanding by observing cosmological redshift prove that the Universe per se is expanding? How do we know what’s happening in parts of the Universe we can’t observe? Couldn’t these parts just as easily be contracting?
   

 

[Ranillon]

1. How can any size be attributed to the Singularity? Given it represents all of existence, what else is it being measured against? Can it somehow be measured against itself internally?
   

1. 
   
   
   I don't think notions of size really worked at the early moments of the Big Bang.
   
   

   Is the Universe that expanded from the Singularity still a Singularity? The internal structure may have changed, but isn’t it still a single entity that represents all of existence?

   
   A duck grows from an egg -- does that mean a duck is an egg? The universe has grown past the point of the singularity.
   
   

   Do we know the size of the Universe now (if so, how), and how do we know that it’s not infinite?

   
   For all practical purposes the universe is as big as we can see since seeing into space is also seeing back into time. If you know the universe is x number of years old then you approach the "end" of the (practical) universe when you detect objects as far away in light years as their have been years.
   
   

   If we don’t know how big the Singularity was, or how big the Universe is, how do we know that the Universe expanded from the Singularity? Couldn’t it always have been the same size and it has just changed internally?

   
   
   As I understand it there is a debate on the size of the universe, but given the extreme effects that the Big Bang would have had on space-time it hard to believe that the universe just expanded into a large empty space waiting
   to fill it. In as-yet-not-proven String Theory a new universe would create its own space rather than growing out into an old one.
   
   

   How does proving that the observable universe is expanding by observing cosmological redshift prove that the Universe per se is expanding? How do we know what’s happening in parts of the Universe we can’t observe? Couldn’t these parts just as easily be contracting?

   
   Anything might be true, but there is no evidence for lumpy universe whose parts expand and contract depending on the location. Everywhere we look we see the red-shift in light. Likewise, the new discovery of Dark Matter and Energy strongly suggests that patterns we can see in actual billions of lightyears away should be in place everywhere. It hard to believe that you can have a different set of physics at work someplace without our universe -- at least, that is, someone comes up with good evidence. Otherwise, it's just speculation.

 

[Dancing David]

There or may not have been a singularity, it is a suggestion of the current models. But they are just models. they work really well after a certain point in the space/time.

We don't know that the universe is not infinite, I can't really explain how it can be infinite and closed at the same time. Due to the event horizon of photons in space time we are limited in what we can see of space time.

The expansion of the universe is an extrapolation of the observed redshift phenomena. Some disagree with the interpretation.

We can't really know what happens in the unobserved universe, isotropy is axiomatic and not proven or disproven (although there are reasons that it seems to be consistent).

 

[Fredrik]

How can any size be attributed to the Singularity? Given it represents all of existence, what else is it being measured against? Can it somehow be measured against itself internally?

The big bang singularity is present in all the solutions of Einstein's equation that describe a homogeneous and isotropic universe. All of those solutions either describe an expanding universe, or a universe with a non-zero density of vacuum. Astronomical observations showed that the universe is expanding (light from other galaxies is redshifted), and that the density of vacuum is at least very close to zero. The original big bang theory is just the claim that our universe can be represented mathematically by one of those solutions.

One of those solutions is easier to understand than the others, so I'll focus on that one. It describes space, at any given time, as a three-sphere with radius R(t), where t is the time coordinate. The time coordinate is always positive in this solution. The function R is increasing, and also goes to 0 as t goes to 0.

That last thing is what physicists really mean when they say that this solution contains a singularity. The singularity is not a point or a region in space-time, so it can't have a size. However, since the "radius" of the universe goes to zero as time goes to zero, the size of the universe can be made arbitrarily small by going far enough back in time. A somewhat sloppy way of saying this would be to say that the size of the universe was 0 at t=0, but there really is no t=0 in this solution. It would be even sloppier to say that the size of the singularity is zero, but I suppose even that would be OK in a conversation between two people who already understand what I've said here.

Is the Universe that expanded from the Singularity still a Singularity? The internal structure may have changed, but isn’t it still a single entity that represents all of existence?

As I said, the time coordinate is always positive in these solutions, so the universe didn't expand from t=0. The universe has never been a singularity in this theory, because there never was a t=0.

Do we know the size of the Universe now (if so, how), and how do we know that it’s not infinite?

We don't know if it's infinite or not.

 

[ynot]

I don't think notions of size really worked at the early moments of the Big Bang.

My question is, do notions of size work at any stage in the existence of the of the Universe?

A duck grows from an egg -- does that mean a duck is an egg? The universe has grown past the point of the singularity.

But was the Universe ever in an “egg” state?

For all practical purposes the universe is as big as we can see since seeing into space is also seeing back into time. If you know the universe is x number of years old then you approach the "end" of the (practical) universe when you detect objects as far away in light years as their have been years.

Our modern, technologically enhanced view of the Universe has allowed us to disprove conclusions that our ancestors made with their limited human vision view. I wonder if future technological advances will similarly disprove our current conclusions. Just because our observational abilities are restricted, it doesn’t mean that truth is restricted, just our ability to find it.

As I understand it there is a debate on the size of the universe, but given the extreme effects that the Big Bang would have had on space-time it hard to believe that the universe just expanded into a large empty space waiting to fill it. In as-yet-not-proven String Theory a new universe would create its own space rather than growing out into an old one.

As I understand it, there is a debate on whether the Universe has a size or not.

Anything might be true, but there is no evidence for lumpy universe whose parts expand and contract depending on the location. Everywhere we look we see the red-shift in light. Likewise, the new discovery of Dark Matter and Energy strongly suggests that patterns we can see in actual billions of lightyears away should be in place everywhere. It hard to believe that you can have a different set of physics at work someplace without our universe -- at least, that is, someone comes up with good evidence. Otherwise, it's just speculation.

If we live inside one “lump”, and are unable to observe beyond that “lump”, I don’t see that it means that other “lumps” don’t exist. Of course without proof it’s just speculation. But it seems to me that it’s speculation that’s possible. Is there a consensus that Dark Matter and Energy have been “discovered”, or are they just theories? I don’t see that contraction is a “different set of physics” from expansion.

 

[Fredrik]

But was the Universe ever in an “egg” state?

I think it's safe to say that the answer is "no". Even in a theory with an initial singularity, the singularity is not a physical state of the universe. It's just a name for the weirdness that happens as time goes to zero. I also think that almost all physicists expect that if a better theory of gravity is ever found, then it won't contain a singularity.

]If we live inside one “lump”, and are unable to observe beyond that “lump”, I don’t see that it means that other “lumps” don’t exist.

I think most theories of inflationary cosmology predict some sort of "lumpy" universe. If you do some googling you can probably find good online articles about it by people like Alan Guth and Andre Linde.

Edit: This is one of them: http://www.stanford.edu/~alinde/1032226.pdf.

Is there a consensus that Dark Matter and Energy have been “discovered”, or are they just theories?

Yes, there's consensus about that.

 

[ynot]

Just had a bit of an “off-the-wall” weird thought - Instead of the currently scenario where space is universally expanding and matter not, could it alternatively be that matter is universally shrinking and space is not?

 

[Dancing David]

My question is, do notions of size work at any stage in the existence of the of the Universe?

Depends on the perspective, from the inside yes, from a conceptual framework of 'out side' :no.

But was the Universe ever in an “egg” state?

Unknowable. Subject to change.

Our modern, technologically enhanced view of the Universe has allowed us to disprove conclusions that our ancestors made with their limited human vision view. I wonder if future technological advances will similarly disprove our current conclusions. Just because our observational abilities are restricted, it doesn’t mean that truth is restricted, just our ability to find it.

yes but there is some suggestion that some questions have no answers.

As I understand it, there is a debate on whether the Universe has a size or not.

Yes and no, current models do say it is closed but likely infinite.

If we live inside one “lump”, and are unable to observe beyond that “lump”, I don’t see that it means that other “lumps” don’t exist.

Quite true but unknowable, except that isotropy does appear to be true for what we can see. So within our lump it seems likely that isotropy applies.

Of course without proof it’s just speculation. But it seems to me that it’s speculation that’s possible. Is there a consensus that Dark Matter and Energy have been “discovered”, or are they just theories?

Theories.

I don’t see that contraction is a “different set of physics” from expansion.

Well it would make for some different expressions of spacetime.

 

[Fredrik]

Just had a bit of an “off-the-wall” weird thought - Instead of the currently scenario where space is universally expanding and matter not, could it alternatively be that matter is universally shrinking and space is not?

If all matter is moving towards something, the universe isn't homogeneous or isotropic. This contradicts observation. What you describe sounds like a spherically symmetric distribution of matter collapsing into a black hole.

The homogeneous and isotropic solutions describe a space-like hypersurface of space-time that it makes some sense to think of as "the universe, at this moment in time". The size of that hypersurface (the function R mentioned previously) is growing with time. The distance between galaxies increases because they stay at the same spatial coordinates while this hypersurface grows.

 

[Fredrik]

Dancing David, we seem to be contradicting each other on the subject of dark matter/energy, so maybe we should clarify our positions. I said that there is consensus about their existence. You said there's not. What I'm trying to say is that even though no one knows what dark matter is, there's lots of observational evidence that shows that it exists. Are you saying there isn't?

I'm also curious about something else you said. What current model describes a universe that's closed but infinite?

 

[Dancing David]

Dancing David, we seem to be contradicting each other on the subject of dark matter/energy, so maybe we should clarify our positions. I said that there is consensus about their existence. You said there's not. What I'm trying to say is that even though no one knows what dark matter is, there's lots of observational evidence that shows that it exists. Are you saying there isn't?

I'm also curious about something else you said. What current model describes a universe that's closed but infinite?

I am just being agnostic on dark matter, but it makes tremendous sense (and seems to be most accurate).

I will see if I can find the source of the closed infinite universe, it may have been on this board.

 

[ynot]

If all matter is moving towards something, the universe isn't homogeneous or isotropic. This contradicts observation. What you describe sounds like a spherically symmetric distribution of matter collapsing into a black hole.

The homogeneous and isotropic solutions describe a space-like hypersurface of space-time that it makes some sense to think of as "the universe, at this moment in time". The size of that hypersurface (the function R mentioned previously) is growing with time. The distance between galaxies increases because they stay at the same spatial coordinates while this hypersurface grows.

The shrinking wouldn‘t be moving matter from it‘s relative positions (spatial coordinates). The centres of two planets would remain the same distance apart, but as they shrink, the distance between each outer surface would uniformally increase. You have to abandon all human perceptions of size. Additional silly thought - As matter shrinks, it “squeezes” out non-matter (light heat etc.) This is just a “brainstorm” and not to be taken too seriously.

 

[Cuddles]

Additional silly thought - As matter shrinks, it “squeezes” out non-matter (light heat etc.) This is just a “brainstorm” and not to be taken too seriously.

What makes you think light isn't matter?

 

[ynot]

What makes you think light isn't matter?

Because matter can’t travel at c.

Why do you think light is matter (if you do)?

 

[fuelair]

For pretty good answers to your questions here, watch The Universe:Beyond the Big Bang. It was on the History Channel the night before you put this up
and will likely show up again.

 

[ynot]

For pretty good answers to your questions here, watch The Universe:Beyond the Big Bang. It was on the History Channel the night before you put this up
and will likely show up again.

Thanks - I live in New Zealand so don’t know when or if it will be shown here. Searched “Beyond the Big Bang“ on youtube and didn’t find it there. Found a couple of “Beyond Einstein” clips that look interesting though.

 

[TuftedPuffin]

My question is, do notions of size work at any stage in the existence of the of the Universe?

Yes. Size could be measured against the Planck length. Though as a singularity, the universe by definition had no size, so your original question is irrelevant.

 

[Cuddles]

Because matter can’t travel at c.

That is not the definition of matter.

 

[ynot]

That is not the definition of matter.

I wasn't claiming that it was the definition of matter, but it certainly is a definition of the difference between matter and light.

You haven’t answered the question regarding whether you think light is matter.

 

[fishkr]

You haven’t answered the question regarding whether you think light is matter.

There is a really famous equation that might be of help here. Begins with an E . . .