# I Expanding universe needs a big bang?

#### CHOP

i have read that it is generally thought to be a consequence of the big bang (so, matter in motion) + something (dark energy) making that matter accelerate. why is the big bang needed in it? can't you just have acceleration?
by the way, i am not sure what is intended by 'prefix' beside the title i used for this post. advanced for advanced responses, i assumed.
thank you

#### mathman

What do you start from?

#### CHOP

Okay, but then, why don't we include "big bang" when accounting for the earth circling the sun? The earth can't circle the sun unless everything 'started' in the first place.
I don't see how the big bang is a contributor to this expansion, because there seems to be nothing to it except that it is accelerating. I mean, the acceleration seems to be not a separate issue to expansion happening (it's one and the same thing). Am I not noticing something?

#### Grinkle

Gold Member
"A" means you want answers that are at the graduate or post-graduate level. I am doubtful that is what you want.

Does the discussion in this thread help?

#### CHOP

Hi Grinkle, thanks for the clarification. Well, we'll see how it goes.

#### CHOP

Grinkle, oh sorry I didn't see what you were linking to. I'll check it out and thank you

#### Bandersnatch

Big bang is the hot and dense early state of the universe that one arrives at when one extrapolates expansion backwards in time. It should not be construed as some ad-hoc creation moment, as it often is depicted in the media, but a straightforward consequence of expansion.

If today the universe is expanding, diluting and cooling, then as you roll back the time you see it contracted, denser and hotter. The somewhat arbitrarily defined earliest (densest and hottest) stages are called Big Bang. Note, that is different than the Big Bang singularity, which is not a physical thing.

#### CHOP

Bandersnatch, I can appreciate all that you have said.
But I don't see how the 'bang' is helping explain what we are seeing today. We are seeing separations increasing in an accelerating way.
I can see how a 'bang' would account for matter moving (radially outward), but at constant velocity. Hence the need for 'dark energy' (something to account for the non constant velocity).
I was thinking about this 'acceleration' feature, and it seems that this feature (all by itself) is sufficient to account for what we are observing. That is to say, it seems that this feature may not be a feature of expansion, but be what expansion is.
Hence my post, asking what exactly is the need to account for expansion with dark energy + big bang, as opposed to simply dark energy?
Grinkle, thanks again. I read that other thread and I do not find it resolves my question.

#### Bandersnatch

There's a lot that could be said about it, but just consider this for now: for approximately half of its history, the universe was decelerating. This is due to how matter and radiation content acts gravitationally on itself, causing recession to slow down. For objects to move at constant velocity, the universe would have to be empty - and it obviously isn't.

There would never be any universe as we know it if there was no initial impulse to expand, as the retarding effect of high density of matter and radiation in the early universe would always overcome the relatively weak accelerating influence from dark energy.

This is easy to appreciate once one takes into account how various densities scale with changing size (aka scale factor, $a$) of the universe. The matter density changes as $1/a^3$, radiation density changes as $1/a^4$, while dark energy density (assuming one, likely, type at least) doesn't change. The first two cause deceleration, the last one causes acceleration.
Today dark energy is just about a bit stronger than matter (and radiation is negligible). It doesn't take much reduction in scale factor to have matter dominate, and with sufficient reduction radiation becomes dominant, both acting to decelerate expansion. So the early universe had to expand at a really high rate to get to where we are.

I.e. some initial expansion is necessary. Furthermore, it is possible to make a stronger argument, that the initial expansion (at whichever time we define the 'initial' to be) had to be in a narrow range of magnitudes, as too much would shut down nucleosynthesis and formation of structures (galaxies etc.) too soon, and too little would lead to quick recollapse - meaning there would be no CHOP to ask such questions beyond either boundary.

#### phinds

Gold Member
I can see how a 'bang' would account for matter moving (radially outward), but at constant velocity.
Well, yeah, if "bang" meant what you seem to think it means, which it does not. The "bang" was not an explosion from a point and in fact the "Big Bang Theory" says nothing about any "bang" but rather just that the universe at the first movement when we can speak meaningfully about it it was a hot dense plasma of possibly infinite extent, with things moving farther apart from each other.

#### Drakkith

Staff Emeritus
2018 Award
But I don't see how the 'bang' is helping explain what we are seeing today. We are seeing separations increasing in an accelerating way.
I can see how a 'bang' would account for matter moving (radially outward), but at constant velocity. Hence the need for 'dark energy' (something to account for the non constant velocity).
I was thinking about this 'acceleration' feature, and it seems that this feature (all by itself) is sufficient to account for what we are observing. That is to say, it seems that this feature may not be a feature of expansion, but be what expansion is.
Unfortunately it is not. The way that objects recede from each other in the expansion of the universe is not like the expansion of fragments away from an explosion. The only way for the two to be the same would be for the big bang to have taken place in a single location, and for that location to have been exactly where our galaxy is located at now. That's a very unlikely coincidence.

#### CHOP

Bandersnatch, agreed about no constant velocity (as you say, gravity slowed recession. For constant velocity space would need be empty). I mentioned constant velocity because I was showing the only way I could fathom the big bang (BB) as actively contributing to this expansion now. That is, I don't see BB as yielding the expansion now. Especially not when we have a thing called dark energy for it.

The 'initial expanding impulse' you mentioned is (I assume) just what BB is. And since it was 'initial' it is no longer active (correct me if wrong). You mention that the early universe had to expand at a really high rate to get to where we are. I assume you mean the BB, the impulse (it got us 'past' where gravity would dominate the situation). That makes sense to me. But this rate is over now, correct? History. So, here we are, and we have an expanding universe (thanks to BB getting us expanded enough to see what dark energy is up to), whereby the separations increase in an accelerated way.

What does BB have to do with today's phenomenon? BB seems to just have 'prepared' it for dark energy to have go, but is not contributing to it. Or maybe you will say it is contributing to it. Okay, but then I ask: What evidence is there for that? Is there any aspect of the expansion today that can't be accounted for by dark energy, and must be accounted for by BB (a BB that is still active)? Or is the reason we talk about BB only because when we trace back we get to BB, so we just 'kept it alive' (so to speak) tracing forward?

Perhaps I do not have a good understanding of what a BB is, as some have mentioned. Very likely I don't. If that's the problem, then can someone tell me how to think (just generally) of the BB, such that I would obtain a sense of how BB could be one of the two contributors to the expanding activity of today? Put another way, if we remove the dark energy contributor (suddenly like a switch), what exactly would we, on that instant, be seeing, as far as the BB contribution?

Thanks for all your help so far,

#### Drakkith

Staff Emeritus
2018 Award
The 'initial expanding impulse' you mentioned is (I assume) just what BB is. And since it was 'initial' it is no longer active (correct me if wrong).
Note that the expansion is due to geometry according to general relativity, not a force. So this 'impulse' should be a setting up of the initial conditions of spacetime that result in expansion, not a 'push' away from everything.

#### CHOP

Drakkith,
right, that's how i am currently understanding it. thanks. the reason i made this post was to find out what is motivating physicists (assuming i have not been misinformed of that motivation) to account for today's expansion with: dark energy + bb (as opposed to dark energy only). Why 2 things, not 1? It seemed to me that dark energy (the accelerating 'causer') is all that is needed. If anyone sees that I am missing something (that I ought to have BB included as a secondary contributor), please let me know and thank you again,

#### phinds

Gold Member
I recommend the link in my signature

#### CHOP

Thanks phinds will check it out now.

#### Ken G

Gold Member
It is certainly true that for the last few billion years, dark energy has been the primary driver of expansion. However, as mentioned already, there needs to be a history that sets up for this possibility, and that's what the "Big Bang" describes. So if you are imagining that the Big Bang kind of "hands off" the expansion to dark energy, perhaps a bit like a runner in a relay race who hands the baton to the next runner, then that's probably not a bad way to think about it. But when you watch a relay race, do you not say that all the runners contribute to the victory in the race? So that's what we mean by the Big Bang contributing to the expansion we see today. (It also set the stage for other key aspects, like the ratio of hydrogen to helium that we see, and the presence of the cosmic microwave background, both of which were present prior to the phase where dark energy started to take over, and indeed we needed the Big Bang to explain those observations even before we knew about any need for dark energy).

Perhaps it would help to better understand how the "handing off of the baton" occurs. Dark energy is a property of empty space, but it is fighting the effects of normal gravity. Hence, dark energy can only dominate over normal gravity (which decelerates expansion as mentioned above) once the matter gets spread out enough that there is enough "empty space" to start making dark energy important. Without that early expansion, dark energy would never come into play, just like the "anchor leg" runner in a relay race would never come into play if the first runner did not hear the starting gun and never began the race in the first place.

#### phinds

Gold Member
@Ken G I think your explanation is slightly misleading because you leave out the fact that the expansion of the universe has been chugging along since the beginning, regardless of dark energy. Dark energy is not what caused the expansion or the continuing expansion, it's what causes the ACCELERATION of that expansion. The expansion itself is just a continuation of the early expansion

#### Ken G

Gold Member
Actually I think that's exactly what my "relay race" analogy makes clear. But I was adding a second paragraph as you posted, it might help. It all depends on what the nature of the issue that CHOP is having, in my view it centers on what he/she interprets "contributing" as meaning.

#### CHOP

Ya this is a great, really polished article. Good stuff. The first way expansion was explained to me, was on a beautiful remote canoe trip, sitting on a high rocky island middle of a large pristine lake, silent. I was young. The baking blueberry muffin was used. Where the berries move from center muffin (but of course no center in space) at the relative rates they do. Good memories. By the way I joined this forum today and I am so impressed by it. So simple and intuitive in its display and navigation, and community outstanding. Friendly, so prompt (wasn't expecting that) and solid, careful, info. Great to have joined. I have one problem with only one thing in the article I'll write it up so I can get it cleared. Thanks again,

#### CHOP

Ken, on glimpse of you post it seems to be 'communicating' with my thinking. I will respond to it after the section of the article I am not jiving with well:

"After inflation, the universe settled down to a more sedate rate of expansion and THIS is what is normally meant by the EXPANSION of the universe"

This 'settling down' took time right? (to be as settled as it is, from as unsettled as it was). So, the expansion of today is that settled down rate, still setting down or finished settling. Which one is it? What is the rate we are now settled to? How was it determined?

So dark energy (DE) is sort of 'hiding' this expansion, making it not knowable via observation alone. Because we can't tell (via observation) what component of the acceleration is DE, and what component expansion. So there must be some disagreement in numbers which causes the conclusion of the existence of DE, correct?

If that's the case then it's a done deal.
But if nothing forces that conclusion, I am left with a feeling that there is too much going on here. Why can't BB (expansion) be all there is? I assume it's because it is thought to be settling or settled (which is not what we see up there). Okay, but then, how did we determine it was settling? If Gravity is why it was settling, why not it pick up now (gravity is less now)?

Can't whatever explains the 'expansion impulse' (I assume it's an accelerated rate) still be present, just no longer settling due to Gravity's role being negligible? Sort of like a bungee jumper accelerates down and then starts settling, but the line breaks during the settling, so she accelerates again.

The problem I see is: The jumper decelerates after acceleration because elastic tension occurs late; in universe case, tension (gravity) does not occur late, since it's always there, so analogy fails. But what explains why the initial gravity didn't bind against the impulse expansion (immediately)? Wouldn't that explanation 'stand for' her initial free fall?

Thanks

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#### CHOP

Ken G,
right, so if it's a hand off then that's in line with what i was thinking and posted. i think you may have hit the nail when you said depends on meaning of contribution. it appears that i may have been interpreted as meaning 'has nothing to do with anything at all'. i understand that we need a few separations for anything to get rolling (whether gravity or DE). But in that sense of 'contribution', why don't we say that BB contributes to any other motion in the universe? We don't mention it, hence (since mentioned) I took 'contribution' to mean 'actively facilitating' this expansion, which i found suspicious. So just to clarify then, the expanding universe today (which is an accelerating one) is being 'caused' by DE, and by nothing else. Correct? And since one poster pointed out that DE is a property of relativity (unless I misunderstood), we'll say that the expanding universe happens for no mystery. If false (there is mystery), what is the mystery?

#### weirdoguy

the expanding universe today (which is an accelerating one) is being 'caused' by DE, and by nothing else. Correct?
Accelerated expansion is caused by dark energy, not the expansion itself. Universe would expand even without it.

#### CHOP

weirdoguy, I don't see how what you said up to your comma is different from what I said up to my comma. For the rest, it seems that you are emphasizing: "although there is accelerated expanding going on right now, there is also something else hiding inside of it, and it is a non accelerated expanding"

If that is what you mean, I do not see what motivates thinking it. If you don't mean that, but mean "There was a non accelerated expanding that happened, called the Big Bang", then okay (I know that's the consensus) but shouldn't the Big Bang have been an accelerated expanding?

#### phinds

Gold Member
... shouldn't the Big Bang have been an accelerated expanding?
Why would you think that? If you set off a stick of dynamite in outer space would you expect the casing pieces to accelerate as they left the center of the blast? What force is it that you think would create that acceleration?

"Expanding universe needs a big bang?"

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