# Flat and accelerating Universe

• oXDawidXo
In summary, the conversation discusses the paradox of the expanding universe, and how the classification of open/flat/closed is not enough to fully understand it. With the discovery of dark energy, it is now believed that the universe is flat and expanding at an accelerating rate. The exact nature and fate of the universe is still being studied and is not yet fully understood.

#### oXDawidXo

Hi!

I'm doing physics project and I'm stuck with a really awful paradox.

Universe appear to be flat (nearly), so it should expand forever (the critical universe). On the other hand we've got acceleration of the universe so open model. Which model is the right one?

oXDawidXo said:
Hi!

I'm doing physics project and I'm stuck with a really awful paradox.

Universe appear to be flat (nearly), so it should expand forever (the critical universe). On the other hand we've got acceleration of the universe so open model. Which model is the right one?
An open universe is not accelerating. In the presence of ordinary matter and radiation, all three geometries-- open, flat, closed -- are decelerating. Closed universes eventually begin to contract again. Flat and open universes expand forever at a decelerating rates.

An accelerating spacetime is not inconsistent with flatness. In fact, accelerating spacetimes are driven towards flatness dynamically.

oXDawidXo said:
Hi!

I'm doing physics project and I'm stuck with a really awful paradox.

Universe appear to be flat (nearly), so it should expand forever (the critical universe). On the other hand we've got acceleration of the universe so open model. Which model is the right one?

It sounds like you are reading some slightly older material. At one point we classified the Universe under the simple 'open/flat/closed' system because we thought the only thing that mattered was the density of matter. If there is not enough, we have an open universe, too much we collapse and just right leads to asymptotic decceleration.

It turns out though that there is more to the Universe. In fact we think most of the energy today is not in the form of matter but 'dark energy' which rather than acting to slow the expansion speeds it up.

Once you include dark energy, the geometry (open/flat/closed) and the fate of the universe (expand forever/collapse) become different questions, it depends on the properties of dark energy which are not yet fully understood.

Our present best guess is that the universe is flat, in that the sum of matter + dark energy density equals the critical density, but that the dark energy dominates and the expansion is accelerating. Out best guess says that this will continue in the future and the expansion will grow ever more rapid, but that question is far from settled.

I think I get it now
Thank you

## 1. What is a "Flat Universe"?

A flat universe refers to the spatial geometry of the universe, specifically the curvature of space. In a flat universe, the overall curvature of space is zero and it is represented by a Euclidean geometry. This means that parallel lines will never intersect and the angles of a triangle will add up to 180 degrees.

## 2. How do we know that the universe is accelerating?

Scientists have observed that the expansion of the universe is accelerating, meaning that the rate at which galaxies are moving away from each other is increasing. This was first discovered through observations of supernovae, which revealed that the light from these exploding stars was fainter than expected. This suggests that the expansion of the universe is accelerating rather than slowing down.

## 3. What is causing the acceleration of the universe?

The cause of the acceleration of the universe is still a topic of ongoing research and debate. One theory is that a mysterious force called dark energy is responsible for the acceleration. Another theory suggests that Einstein's theory of gravity, known as General Relativity, may need to be modified to better explain the acceleration.

## 4. How does the concept of a flat and accelerating universe impact our understanding of the cosmos?

The concept of a flat and accelerating universe has significant implications for our understanding of the cosmos. It suggests that the universe will continue to expand at an accelerating rate, eventually leading to a "Big Freeze" scenario where all matter in the universe becomes too far apart for any significant interactions. It also challenges our current understanding of gravity and the fundamental laws of physics.

## 5. Is there any evidence that supports a flat and accelerating universe?

Yes, there is a considerable amount of evidence that supports the idea of a flat and accelerating universe. In addition to the observation of supernovae, scientists have also studied the cosmic microwave background radiation, the leftover radiation from the Big Bang. This radiation shows a pattern of fluctuations that align with the predictions of a flat universe. Other evidence includes measurements of the large-scale structure of the universe and the behavior of galaxies and galaxy clusters.