Expanding Universe

tgramling

For those of you accepting singularity,

When the big bang occured there was a massive amount of energy exerted.
Did this energy exert a positive acceleration on everything, meaning that it will continue to expand forever? Or did it have a negative acceleration, like throwing a ball up in the air here on earth

blitz.km

A positive acceleration..
because we have learnt that the galaxies are continuously moving away from each other.

Molydood

Isn't it that the bang was positive and gravity is negative?

tgramling

Thank you for the relpies, sorry for the late response. But havn't we also learned that the universe has contracted before? I think it was just small fluctuations. If this is true then what exactly caused it to do this?
Could it be possible that the universe is just in its expanding state.
Like the throwing a ball example, could it just be that the big bang exploded and sent everything out on a negative acceleration, and that it just hasnt reached the peak yet?

Nabeshin

Fifteen years ago, nobody would have been able to answer this for you!

It is entirely possible to have a cosmology which initially expands, but at a decelerating rate, and eventually recollapses. Or, alternatively, expands at a decelerating rate, but never quite recollapses and simply levels out. These types of universes are dominated by matter, radiation, or some combination of the two. And like I said, up until about ~1997 we believed we lived in one of these universes.

However, we now know that our universe is both expanding and accelerating, and as such, will continue to expand forever, at an ever increasing rate. The culprit is what we call dark, or vacuum, energy.

I haven't heard anything from mainstream cosmology saying that the universe contracted in the past, but I don't exactly follow the literature as closely as I probably should.

Blitz.km: While true that galaxies are moving away from each other, this only shows the universe is expanding, not contracting. Expansion does not imply acceleration, however, and while your conclusion is correct, it is based on incorrect reasoning.

My.Daddy

You are talking as if the reason for accelerated expansion is really confirmed. Why don't you give it a break and set some standards for acceptance of an explanation.

Can you even tell me how exactly is universe expanding, the space expansion?

Take four points in space A, B, C, D all placed at equal distances from each other (to start with), so as space expands between B & C, B moves away from C and vice versa, however when space expands between A & B, B is pushed back to its original position. OMG!

tgramling

Thanks for the replies, but Daddy brings up a good point. How do we really know how the universe is expanding, like Nabeshin said,

What undeniable evidence changed our minds in 1997? How do we, "know that our universe is both expanding and accelerating," now? If it is "entirely possible" to have an expanding/collapsing universe, what shows us that we don't?