Big Bang questions

[AmherstVA]

How do you know that the item that 'exploded' to cause the Big Bang doesn't have a birth to death life just like stars do? How do you know it didn't just erupt the universe in one phase of many? How do you know it isn't still spewing out 'universes' or other types of activity?

[antoniseb]

How do you know that the item that 'exploded' to cause the Big Bang doesn't have a birth to death life just like stars do? How do you know it didn't just erupt the universe in one phase of many? How do you know it isn't still spewing out 'universes' or other types of activity?

Welcome to the BAUT forum AmherstVA.

To answer your question with a question, how can we in our one universe know what's happening in other universes (if they exist at all)?

[JustAFriend]

Your question is exactly why people become cosmologists (pro or amateur).

We don't know the answers and we're still looking for the right questions....

[Ken G]

One thing to add-- the "Big Bang" was not an explosion, though sometimes it works to picture it that way. The reason it was not an explosion is that explosions work by having a high pressure region surrounded by a low pressure region, and the high pressure region expands into the low pressure region. The "space" in which that all happens is just a bystander. But the Big Bang model is different because it is based on general relativity, which is a theory of gravity not of explosions, and because it happens everywhere equally (according to the cosmological principle, the core simplifying principle the Big Bang model uses). Since it happens everywhere, there is no pressure difference that can create an explosion. Since it is ruled by general relativity, you can think of space itself as a player in the "explosion", as if space itself were what was exploding. This does not mean we have an actual theory of what space is doing, it's just a useful picture often invoked in the Big Bang model, that space itself is expanding according to some unknown initial conditions and controlled by gravity. What the picture depends on is the way we choose to coordinatize space, which determines our language about what space is doing, moreso than anything real that space is actually doing. Perhaps someday we'll have a consistent theory about what space is actually doing, but for now it is only an arbitrary construct that lives within the greater manifold called spacetime (which is itself just a construct, of course, but so it is with science).

[Brian T]

I'm just an undergrad working towards a degree to teach science, so take this for what it's worth. I'll explain as best as I can, but I'm no professional. In regards to the first question:

How do you know that the item that 'exploded' to cause the Big Bang doesn't have a birth to death life just like stars do?

In the 1920's Edwin Hubble figured out that space is expanding. Cosmologists thought that if gravity is strong enough to counteract the expansion from the Big Bang, the whole Universe might collapse in on itself to form a Big Crunch. But we found the answer to that question in the mid 1990's. At that point cosmologists discovered that not only is the universe expanding, but it's speeding up. No one, and I mean absolutely no one suspected that would be the case. It flies in the face of everything we thought could have been possible.

As to what's causing this, that's where unseen mass and force (dark matter and dark energy) come into play. These ideas are still in their infancy, so we're basically fumbling in the dark trying to make sense of them.

The picture painted by the evidence thus far is, the Universe looks like it will keep expanding faster and faster. Eventually, space will grow faster than light can spread between the stars, and once every star in the Universe exhausts its fuel, all of space and time will be nothing but black emptiness. No more stars, galaxies, nebulas, nothing.

Kind of a depressing thought, but it won't happen for quite a few billion years from now. We can enjoy the view while we're here. I hope this helps.

[Jeff Root]

At that point cosmologists discovered that not only is the universe
expanding, but it's speeding up. No one, and I mean absolutely no one
suspected that would be the case. It flies in the face of everything we
thought could have been possible.

I should have suspected it. I have an hypothesis involving antigravity
that predicts acceleration of the expansion. But I didn't think it through
and failed to realize that it makes that prediction, until after the
acceleration had been discovered and announced in 1998. It might
only be a coincidence, though.

As to what's causing this, that's where unseen mass and force (dark
matter and dark energy) come into play. These ideas are still in their
infancy, so we're basically fumbling in the dark trying to make sense of
them.

The picture painted by the evidence thus far is, the Universe looks like
it will keep expanding faster and faster. Eventually, space will grow
faster than light can spread between the stars, and once every star in
the Universe exhausts its fuel, all of space and time will be nothing but
black emptiness. No more stars, galaxies, nebulas, nothing.

That is the scenario for one explanation of dark energy. My hypothesis
includes a different explanation, which predicts the acceleration of
the expansion to reduce over time, not increase. It will still continue
to accelerate, according to my hypothesis, but at an ever-decreasing
rate. And there are other hypotheses which also do not predict an
increasing rate of acceleration.

-- Jeff, in Minneapolis

[Kwalish Kid]

But we found the answer to that question in the mid 1990's. At that point cosmologists discovered that not only is the universe expanding, but it's speeding up. No one, and I mean absolutely no one suspected that would be the case. It flies in the face of everything we thought could have been possible.

That's very much not true, since the physics behind the acceleration had been around for about 70 years when the measurement evidence came in. Those who believed that there is a quantum vacuum energy that takes part in gravity would predict this sort of acceleration.

[Cougar]

Those who believed that there is a quantum vacuum energy that takes part in gravity would predict this sort of acceleration.

That is certainly the implication of supposing Lambda>0, which apparently more than a few were playing around with....

[dhd40]

I have an hypothesis involving antigravity
that predicts acceleration of the expansion.
..... And there are other hypotheses which also do not predict an
increasing rate of acceleration.

-- Jeff, in Minneapolis

(my bold)
What´s actually the meaning of "accelerated expansion (of the Universe)"?
Is it dv/dt > 0 or d²v/dt² = da/dt > 0?
t=time, v=speed (or velocity), a=acceleration

[Spaceman Spiff]

(my bold)
What´s actually the meaning of "accelerated expansion (of the Universe)"?
Is it dv/dt > 0 or d²v/dt² = da/dt > 0?
t=time, v=speed (or velocity), a=acceleration

It means the second Friedmann equation here has a positive value, where a(t) is the dimensionless scale factor of expansion, and so (1/a)da/dt is the Hubble parameter, and the second derivative measures the acceleration term. Very early on the cosmological constant term was abandoned as extraneous, and so for decades it was presumed that second time derivative of a(t) is less than zero. See also here.

[Jeff Root]

In simple terms, your basic, off-the-shelf expansion is constant with time.
It isn't constant with distance, which is tremendously important, but I'll just
ignore that since it isn't what I'm talking about here.

What was assumed to be happening was that gravity has been slowing the
expansion-- that is, decelerating it. So that over time, the rate of expansion
would be decreasing.

However, in 1998 it was discovered that the expansion is accelerating, that
is, the rate of expansion is increasing over time. So that everything moves
apart faster and faster.

Some theories say that this acceleration is increasing with time. They say
that the acceleration used to be very low, or zero, is at a moderate level
now, and will eventually be very high. So that not only is the Universe
expanding, and not only is it expanding faster and faster, but it is expanding
faster and faster, faster and faster!

My own speculation which involves gravitational repulsion predicts that
the expansion will continue to accelerate, but at an ever-decreasing rate,
so that the Universe is expanding faster and faster, slower and slower.

I drew a line on someone else's graph, which might help a little bit:

http://www.freemars.org/jeff/misc/cosmolog/expansn1.png

-- Jeff, in Minneapolis

[OldAstrofan]

How do you know that the item that 'exploded' to cause the Big Bang doesn't have a birth to death life just like stars do? How do you know it didn't just erupt the universe in one phase of many? How do you know it isn't still spewing out 'universes' or other types of activity?

For the first question i might suggest a "heat death", the "time"(or should i say stage of the universe) when all the reactions (physical, chemical, nuclear, gravitational(?)) will go to a stop because of the second law of thermaldynamics. The entropy of the universe will only increase.

*As a side thought: does gravitational effects caues the rise in entropy?

For multiverses it is still currently a prospect with no actual observation to support. Think you may be interested in the conclusions of string theory. Check it out! Just type string theory in any search engine and you will get tons of it.