# What caused cosmic inflation?

## Main Question or Discussion Point

Not long after the Big Bang, space expanded faster than light for a brief amount of time, slowed down, then sped up again.

Is there an answer to this question?

berkeman
Mentor
Not long after the Big Bang, space expanded faster than light for a brief amount of time, slowed down, then sped up again.

Is there an answer to this question?
References please. You know how this works here at the :PF.

George Jones
Staff Emeritus
Gold Member
Not long after the Big Bang, space expanded faster than light for a brief amount of time, slowed down, then sped up again.

Is there an answer to this question?
To what question do you refer?

My guess is that you want to know about two questions.

1, What caused inflation, the early rapid expansion?

2. What is now causing the accelerating expansion?

There are educated guesses, but there are no firm answers to either question.

Not long after the Big Bang, space expanded faster than light for a brief amount of time, slowed down, then sped up again.

Is there an answer to this question?
Yep. that's apparently what happened based on observations, and reasonable maths.
As far as I know there isn't yet any theory which would explain it.
Dark energy is called dark because we don't really have much of a clue about what it is.

kimbyd
Gold Member
Not long after the Big Bang, space expanded faster than light for a brief amount of time, slowed down, then sped up again.

Is there an answer to this question?
This isn't an accurate description. In the standard model of cosmology, which includes inflation in the early universe, the rate of expansion has always been decreasing (and faster-than-light expansion is a nonsensical statement with no meaning, as expansion is not a speed: it's like saying a car is traveling faster than 1MHz).

But there isn't any known cause for inflation at the moment. Inflation itself isn't entirely certain (there are a few alternative models around that also match the data).

PeterDonis
Mentor
2019 Award
In the standard model of cosmology, which includes inflation in the early universe, the rate of expansion has always been decreasing
This might be worth a bit of clarification. The Hubble constant $H$ has always been decreasing. However, the quantity $\ddot{a} / a$ (roughly, the dimensionless second time derivative of the scale factor) has not; it was increasing during inflation, decreasing after the end of inflation until a few billion years ago, and since then has been increasing again (though much, much, much more slowly than during inflation).

PeterDonis
Mentor
2019 Award
it was increasing during inflation, decreasing after the end of inflation until a few billion years ago, and since then has been increasing again
Correction: the "acceleration" $\ddot{a} / a$ was positive during inflation, negative from the end of inflation until a few billion years ago, and has been positive (though much, much, much smaller than during inflation) since then. By contrast, $\dot{H}$, the time derivative of the Hubble constant, has always been negative.

This may or may not be pop science but Alan Guth in his lectures “Inflationary Cosmology: Is Our Universe Part of a Multiverse” says that inflation was caused by a patch of repulsive gravity-bearing matter in the pre-inflationary universe.

I suppose that this is also speculative, or is it the more-or-less accepted consensus in cosmological circles today?

IH

PeterDonis
Mentor
2019 Award
Alan Guth in his lectures “Inflationary Cosmology: Is Our Universe Part of a Multiverse” says that inflation was caused by a patch of repulsive gravity-bearing matter in the pre-inflationary universe
Which particular lecture? There are a lot of them (assuming you mean the video lectures on the MIT open courseware site).

Which particular lecture? There are a lot of them (assuming you mean the video lectures on the MIT open courseware site).

This is the one at around 13:20:

The slide showing that he is commenting says “The combination of GR and modern particle theories predicts that, at very high energies, there exists forms of matter that create a gravitational repulsion!”

He continues talking about a patch of such repulsive gravity material as being responsible for inflation at 13:45...

IH

PeterDonis
Mentor
2019 Award
The slide showing that he is commenting says “The combination of GR and modern particle theories predicts that, at very high energies, there exists forms of matter that create a gravitational repulsion!”
Ok. He is using the term "matter" rather loosely; what he is really talking about is a "false vacuum" state of a quantum field (the inflaton field) which has similar properties to a large positive cosmological constant, i.e., it causes exponential expansion. But it is not a "material" in any ordinary sense of the word.

As for whether it's just speculative, inflationary cosmology is more than just speculative since it has made some predictions about observable quantities, such as the particular form of variations in the CMBR, which have so far matched observations. But it doesn't currently have the same confidence level as, for example, our model of the universe from the Big Bang (i.e., the end of inflation according to inflationary cosmology) onward.

In the Guth video, at about 57:30 he is talking about local decay of eternal inflation that results in the multiverse "pocket universes", and he clarifies that "eternal" inflation means "forever into the future" only... that "inflation would start at some finite time".

Why? If the inflation is able to produce "pocket universes" forever, it seems to me that its properties that allow that must be continuous. I'm not seeing why inflation can't have always been happening. He does not elaborate on why it's only into the future... it would seem very attractive and elegant if the inflation was eternal both past and future, so there must be some reason why not. Anyone know?

In may text books it is stated that inflation happens after the big bang but this may be mistake. It might be better to say inflation happens before the big bang. See Guth 30:52 into this video:

In the Guth video, at about 57:30 he is talking about local decay of eternal inflation that results in the multiverse "pocket universes", and he clarifies that "eternal" inflation means "forever into the future" only... that "inflation would start at some finite time".

Why? If the inflation is able to produce "pocket universes" forever, it seems to me that its properties that allow that must be continuous. I'm not seeing why inflation can't have always been happening. He does not elaborate on why it's only into the future... it would seem very attractive and elegant if the inflation was eternal both past and future, so there must be some reason why not. Anyone know?
This issue is also addressed in the video I posted.

This isn't an accurate description. In the standard model of cosmology, which includes inflation in the early universe, the rate of expansion has always been decreasing (and faster-than-light expansion is a nonsensical statement with no meaning, as expansion is not a speed: it's like saying a car is traveling faster than 1MHz).

But there isn't any known cause for inflation at the moment. Inflation itself isn't entirely certain (there are a few alternative models around that also match the data).
"...expansion is not a speed..." What does inflation / expansion mean in this context? Also, has the standard model for the big bang been discredited and replaced by another model? If so, which model has replaced it. I'm a novice so please keep your answer novice-friendly if you can.

"...expansion is not a speed..." What does inflation / expansion mean in this context? Also, has the standard model for the big bang been discredited and replaced by another model? If so, which model has replaced it. I'm a novice so please keep your answer novice-friendly if you can.
Speed is the rate at which something moves through space with respect to another object but that doesnt have any meaning when it comes to the expansion of the universe. The expansion of the universe has a rate but it doesnt have a speed, see here for more details.
http://www.preposterousuniverse.com/blog/2015/10/13/the-universe-never-expands-faster-than-the-speed-of-light/

The problem is the phrase "standard big bang model" doesnt have a well defined meaning again this is discussed in the video I posted. I seriously suggest you watch that many of yoru questions are answered by the leading experts in the field.

PeterDonis
Mentor
2019 Award
In may text books it is stated that inflation happens after the big bang but this may be mistake.
It is. Guth explains the correct model well in the video.

It is. Guth explains the correct model well in the video.

Agree with two exceptions though. Slightly after that bit about a patch of repulsive gravity material he sets out that this is possible without violating the conservation of energy because the energy budget of the universe as a whole is zero, or very close to zero.

This is in his slide # 6 with his generalised “Miracle of Physics #2”: The energy of a gravitational field is zero. In this way, he says that the positive energy of all matter and radiation in the universe is balanced by the negative energy of gravity. I don’t quite understand this if an object in a gravitational field has (positive) energy. Does he mean that the ‘creation’ of a gravitational field involves negative energy?

The second exception is that he does not make clear enough how the energy of inflation is distinct from that of Dark Energy and how physicists are certain of this distinction, certain that Dark Energy is not in some way a relic of the energy of inflation.

IH

In this way, he says that the positive energy of all matter and radiation in the universe is balanced by the negative energy of gravity. I don’t quite understand this if an object in a gravitational field has (positive) energy. Does he mean that the ‘creation’ of a gravitational field involves negative energy?
Someone can clarify better than me, but I think gravity energy is considered negative by imagining that if a small mass were an infinite distance from a large mass, the gravitational pull upon it would be zero, and for any position only some finite distance from the large mass, there would need to be positive energy required to get the small mass out to infinity... so a negative value needs the addition of positive value to get to zero.

This is similar to the old hifi volume controls that showed 0dB at maximum and all lower volumes were indicated by increasingly negative dB numbers...

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