# Universe expansion, then deceleration, then acceleration?

• philhar
In summary: On the other hand, matter is not constant - it can be converted into different forms of energy and can also interact and merge with other matter, changing its density. This is why matter is variable while dark energy remains constant.What is meant by "dark energy decays"?This phrase is often used in alternative theories to describe the idea that the density of dark energy may change over time. However, in the standard LCDM model, dark energy is assumed to remain constant, so this term does not apply.
philhar
My first post here.
My level of expertise in this field = 0, although my interest is very high!

If I understand well what I've been reading:

after the BB the universe starting expanding
then the expansion slowed down
then for the last 5 billion years the expansion reversed its course and started to
accelerate.

Why?
AND
Could the expansion slow again and even result in "the big crunch"?

philhar said:
...then for the last 5 billion years the expansion reversed its course and started to
accelerate.

Why?

there is a standard explanation for this in terms of the prevailing LCDM model.
one can think up other explanations, but I personally don't pay them much heed. The standard LCDM makes good enough sense.

the explanation is simple. dark energy is assumed to have a constant density which starts off very small compared with matter density (measured as it's energy equivalent to have everything in the same terms)
BUT as space expands the matter density becomes less and less!
Finally dark energy becomes the dominant form, just by staying constant as matter thins out.

So the dark energy accelerative effect prevails.

While matter dominates, expansion slows, then when dark energy dominates, percentagewise, expansion speeds up.
=================================
if you want more detail, look up Friedmann equation in wikipedia

the equations that all cosmology is based on are the two Friedmann equations which control the evolution of the scale factor a(t) in the Friedmann Robertson Walker metric.

the first Friedmann equation says that the the sign of the second time derivative a" (t) is determined by the expression
-(rho(t) + 3p(t)) where rho is the average energy density and p is the pressure

matter typically has zero pressure and DE has negative pressure, so while matter is dominant this term is mostly just minus rho(t).
So the expression is negative and a'(t) decreases

but after a while the important part is the negative p(t) and minusminus is plus, so the expression becomes positive and a'(t) increases.
======================

According to the standard LCDM model, collapse is not in the cards. Dark energy is constant, the pressure it exerts is constant. expansion continues gradually accelerating indefinitely.

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Thank you!

Acceleration of the universe is expanding space. How then does gravitation and dark energy play a role? Gravitation attracts matter while dark energy repulses it. Space is not matter (in the non-quantum sense). It should not be effected by either (other than curviture). Gravitationaly bound objects would appear to be static in expanding space but would be moving though space thus have velocity. Dark energy would give the look of expansion to an observer but would also create velocity. Objects in expanding space should be comoving but have no velocity (if you take away their peculiar motion). Space is expanding and we are along for the ride i.e. the rubber sheet universe model. The way I figure it, if dark energy has anything to do with acceleration of expanding space it is creating more of it. How do you see it?

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marcus said:
BUT as space expands the matter density becomes less and less!
Finally dark energy becomes the dominant form, just by staying constant as matter thins out.

So the dark energy accelerative effect prevails.

While matter dominates, expansion slows, then when dark energy dominates, percentagewise, expansion speeds up.

Now the universe is accelerating, how can matter dominate again?
If the accelerated expansion occurs, then how the density of matter can increase again?

Atran said:
Now the universe is accelerating, how can matter dominate again?
If the accelerated expansion occurs, then how the density of matter can increase again?
It might not, Atran. The universe could go right on accelerating forever. However, if the dark energy decays away, then matter will once again rule the cosmos.

Yes, you just misread marcus' post. Put his last sentence in your quote on top.

Atran said:
Now the universe is accelerating, how can matter dominate again?
If the accelerated expansion occurs, then how the density of matter can increase again?

I'm not sure I understand your problem. For a long time, for billions of years, matter dominated, and the expansion was gradually slowing down.
Then the matter density got low enough that dark energy began to dominate, and the expansion started to speed up.

It seems straightforward. I guess it's possible you misunderstood something and Ich may have correctly identified your confusion. But I'm not sure.

In the standard model (socalled LambdaCDM) we do not assume matter will ever dominate again, and we do not assume that the dark energy "decays" or changes in any way. It always stays at a constant density of about 0.6 nanojoule per m^3 and a constant negative pressure of -0.6 nanopascal. There is no foreseen "big rip" or "big crunch" or any other of these exciting prospects.

According to the standard model, matter will not dominate again, so I know what you mean now.

Another problem I have: The expansion accelerates, so what happens if the speed of the expansion gets near the speed of light? I know that nothing can reach the speed of light.

========================================

Any idea of why dark energy is constant and matter variable?
What is meant by "dark energy decays"?

Thanks for the replies.

Atran said:
Another problem I have: The expansion accelerates, so what happens if the speed of the expansion gets near the speed of light? I know that nothing can reach the speed of light.
Even in nonaccelerating spacetimes, objects can move apart at speeds surpassing that of light. This is because the speed at which two objects move apart is proportional to their separation: v = Hr, where v is their recession velocity, r is there separation, and H is a constant known as the Hubble constant. You can see that at r = c/H, their recession velocity is c. There is no contradiction with relativity; the objects themselves are locally at rest. It's the expansion of space that causes them to move apart, and special relativity puts no bounds on the rate of spatial expansion.
Any idea of why dark energy is constant and matter variable?
In the standard model, LCDM, the dark energy is provided by a cosmological constant -- the energy of the vacuum. As its name indicates, it possesses a constant energy density. Matter has a variable density because it dilutes as the universe expands. The energy of the vacuum is a very different substance indeed.
What is meant by "dark energy decays"?
Contrary to LCDM, in which the dark energy is constant, there has also been lots of work on models in which the dark energy is a function of time. By 'decays' I really mean the energy density decreases; I don't strictly mean that the dark energy undergoes any decays in the particle physics sense, although this is possible. It's possible to hypothesize an energy density that decreases as the universe expands, however, at present there is no observational evidence for dark energy that does anything else other than remain constant.

philhar said:
My first post here.
My level of expertise in this field = 0, although my interest is very high!

If I understand well what I've been reading:

after the BB the universe starting expanding
then the expansion slowed down
then for the last 5 billion years the expansion reversed its course and started to
accelerate.

Why?
AND
Could the expansion slow again and even result in "the big crunch"?

how we know that universe was decelerating before 5 billion years can anybody tell me this

umeshbilagi said:
how we know that universe was decelerating before 5 billion years can anybody tell me this

I think it has been accelerating for at least 5 billion years. I don't think anyone said acceleration started then. One cannot say with precision. Acceleration may have started when the expansion process was 7 billion years old (and so then it would have been going on now for 6.7 billion years.)

How we know? There are various ways that cross check each other. Different ways, if they are consistent, reinforce and corroborate each other.

I will tell you how I calculate when acceleration started.

The present universe data is that (approximately) the dark energy density is 73% of total and (dark and ordinary) matter is 27%. For decel to stop and accel to begin it must be the case that dark energy reaches 1/3 of total. It must be at least HALF as abundant as matter.

So think of a cubic meter of space having 73 units of DE and 27 units of matter. Twice 73 is 146, that is how much matter we can have per cubic meter when accel starts. What is 146/27? It is 5.4. What is the cube root of 5.4? 1.75.

That means that if we go back to a time when distances were shorter by a factor of 1.75, then volumes will be reduced by a factor of 5.4, and the matter density will be increased by a factor of 5.4 and it will be 146 units per cubic meter. Then DE is 73 and matter is 146, and accel can start.

Now the redshift that corresponds to distances shorter by factor 1.75 is redshift z = .75. I have calculated that if I look thru a telescope and see a galaxy whose light is redshifted z = 0.75 then the people I would see living on that galaxy (with a better telescope) are living at a time when acceleration is just beginning. The z=0.75 galaxy which you see is existing in the era when decel stopped and accel. began.

================
If you google "wright calculator" you will get the online calculator that tells how old the universe was at the z=.75 time. It says 7.1 billion years.
Just type in .75 and it will tell you.

Therefore according to my quick calculation I would say that, as of today, accel has been going on for 6.6 billion years.

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marcus said:
I think it has been accelerating for at least 5 billion years. I don't think anyone said acceleration started then. One cannot say with precision. Acceleration may have started when the expansion process was 7 billion years old (and so then it would have been going on now for 6.7 billion years.)

How we know? There are various ways that cross check each other. Different ways, if they are consistent, reinforce and corroborate each other.

I will tell you how I calculate when acceleration started.

The present universe data is that (approximately) the dark energy density is 73% of total and (dark and ordinary) matter is 27%. For decel to stop and accel to begin it must be the case that dark energy reaches 1/3 of total. It must be at least HALF as abundant as matter.

So think of a cubic meter of space having 73 units of DE and 27 units of matter. Twice 73 is 146, that is how much matter we can have per cubic meter when accel starts. What is 146/27? It is 5.4. What is the cube root of 5.4? 1.75.

That means that if we go back to a time when distances were shorter by a factor of 1.75, then volumes will be reduced by a factor of 5.4, and the matter density will be increased by a factor of 5.4 and it will be 146 units per cubic meter. Then DE is 73 and matter is 146, and accel can start.

Now the redshift that corresponds to distances shorter by factor 1.75 is redshift z = .75. I have calculated that if I look thru a telescope and see a galaxy whose light is redshifted z = 0.75 then the people I would see living on that galaxy (with a better telescope) are living at a time when acceleration is just beginning. The z=0.75 galaxy which you see is existing in the era when decel stopped and accel. began.

================
If you google "wright calculator" you will get the online calculator that tells how old the universe was at the z=.75 time. It says 7.1 billion years.
Just type in .75 and it will tell you.

Therefore according to my quick calculation I would say that, as of today, accel has been going on for 6.6 billion years.

in this we have taken that dark energy is responsible for acceleration of universe and made calculation but how accept that dark energy is responsible for acceleration. decay of present matter and appearance of dark matter how is this dark matter being appearing with time can i know this

## 1. How do we know that the universe is expanding?

Scientists have observed that distant galaxies appear to be moving away from us at an accelerated rate. This is known as the redshift phenomenon, where light from these galaxies is shifted towards the red end of the spectrum, indicating that they are moving away from us. This observation supports the theory of an expanding universe, where the space between galaxies is getting larger.

## 2. What caused the deceleration of the universe's expansion?

The deceleration of the universe's expansion is believed to be caused by the gravitational pull of matter in the universe. As the universe expands, the gravitational force between galaxies acts as a braking mechanism, slowing down the rate of expansion.

## 3. What is responsible for the acceleration of the universe's expansion?

The acceleration of the universe's expansion is believed to be caused by dark energy, a mysterious force that makes up about 70% of the universe's total energy. This force is thought to have a repulsive effect, counteracting the gravitational pull of matter and causing the expansion of the universe to accelerate.

## 4. How does the expansion, deceleration, and acceleration of the universe affect us on Earth?

The expansion of the universe does not have a noticeable effect on us on Earth, as the distances between galaxies are so vast. However, the accelerating expansion of the universe may have long-term consequences, such as the eventual separation of other galaxies from our own. Additionally, the study of the universe's expansion can help us understand the origins and fate of the universe.

## 5. Will the universe continue to expand forever?

The current scientific consensus is that the universe will continue to expand forever. However, there are some theories, such as the Big Crunch theory, which suggest that the expansion may eventually slow down and reverse, causing the universe to collapse in on itself. Further research and observations are needed to fully understand the fate of the universe.

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