The Inflationary and The Big Bang Theory

[yazanhomsi]

Hello everyone,

My question is: The big bang model proposes that the rate at which the universe is expanding is now decelerating. The inflationary theory proposes that the speed of expansion and separation started out slow and now it is accelerating.
But as we know, recently it has been found that the rate at which the universe expands is actually ACCELERATING (dark energy might possibly be the reason for that).
Does that mean the big bang theory is wrong? I know the inflationary theory was introduced to modify the big bang theory. Can someone explain this to me?

However we should keep in mind that the big bang theory predicted that there are remnant photons from the creation event permeating space (cosmic microwave background radiation), which turned out to be correct.

So we cannot say that the big bang is wrong.

If someone can support me with a detailed explanation it would be great.

Thank you in advance.

 

[Orodruin]

The big bang model proposes that the rate at which the universe is expanding is now decelerating.

No it does not. This is true if the Universe contains only matter and radiation, but not in general.

The inflationary theory proposes that the speed of expansion and separation started out slow and now it is accelerating.

No, you are mixing up inflation with dark energy. They are not the same although they require similar conditions, i.e., a dominant component of dark energy of some sort.

Does that mean the big bang theory is wrong?

No.

I know the inflationary theory was introduced to modify the big bang theory. Can someone explain this to me?

Inflation was introduced to explain the apparent homogeneity of the initial conditions for the big bang (among other things). Without it, it becomes difficult to explain why the CMB is so homogeneous and why the Universe seems so flat.

 

[Chalnoth]

Hello everyone,

My question is: The big bang model proposes that the rate at which the universe is expanding is now decelerating. The inflationary theory proposes that the speed of expansion and separation started out slow and now it is accelerating.
But as we know, recently it has been found that the rate at which the universe expands is actually ACCELERATING (dark energy might possibly be the reason for that).
Does that mean the big bang theory is wrong? I know the inflationary theory was introduced to modify the big bang theory. Can someone explain this to me?

As Orodruin mentioned, the old view that the big bang theory had was that we only had matter and radiation in our universe. The big bang theory predicts a relationship between the matter/energy content of our universe and how the rate of expansion changes over time. But it doesn't make any predictions about what the contents of the universe are, so no, the accelerated expansion we observe today does not challenge the big bang theory at all. It just suggests different matter/energy contents of our universe (hence dark energy).

Inflation also doesn't offer any challenges for the big bang theory, as it's just different matter/energy contents again.

Now, the big bang theory also assumes that General Relativity is completely accurate. We know this cannot be true because quantum mechanics and general relativity do not agree with one another, and not knowing the correct theory of quantum gravity means we can't be sure how well this assumption holds for the early universe. From this, we know that the big bang theory is wrong at some level, though probably only during the very early universe.