# Tag Info

Accepted

### What are the alternative theories of dark energy? ($w \neq -1$)

Measuring $w$ is actually what I do for a living. The current best measurements put $w$ at $-1$ but with an uncertainty of $5\%$, so there's a little room for $w \ne -1$ models, but it's not big and ...
• 1,790

### Why is FTL travel impossible if the universe expands FTL?

The expansion of the universe is not measured in units of speed, so it cannot really be compared to c in the first place. Saying that it is faster than the speed of light is “comparing apples and ...
• 65.6k
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### Why do we interpret the accelerated expansion of the universe as the proof for the existence of dark energy?

The accelerated expansion of the universe is not direct evidence for dark energy, i.e. a perfect fluid contribution to the stress-energy tensor with $w = -1$. Dark energy is just by far the simplest ...
• 95.3k
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### How did the universe shift from "dark matter dominated" to "dark energy dominated"?

The title and the text actually ask two different questions. While Kyle Oman and Thriveth answer the title excellently, I'll address the question in the text which asks "Why did the Universe expand in ...
• 10.4k
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### Why is dark energy dominant between galaxies but not inside galaxies?

These aspects of astronomy and cosmology are indeed very interesting and very significant, but don't allow the names to get in the way of your understanding. Dark matter is a form of matter made (most ...

### How did the universe shift from "dark matter dominated" to "dark energy dominated"?

Let's start partway through the expansion of the Universe in the matter dominated epoch. At this time the energy density is dominated by matter, but the dark energy and radiation components are still ...
• 18k

### So Universe Expansion is accelerating, it already did with the Big Bang, so what came in-between?

The expansion of the Universe is driven by the dominant forms of energy and matter density. This is essentially an expression of a famous statement by John Wheeler (from memory, apologies if I get the ...
• 34.3k

### Why is FTL travel impossible if the universe expands FTL?

Compare this to an ant walking on a very stretchy rubber band. The ants walking speed is limited, it can never exceed a certain speed (let's call it $c$). Now if we continuously stretch the rubber ...
Accepted

### Could dark energy just be particles with negative mass?

If dark energy would consist of particles, it would dilute with the growing radius of the universe to the third power, since the total number of particles would stay the same while the volume ...
• 7,737
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### If gravity is attractive, why doesn't the Milky Way contract?

Both energy and angular momentum must be conserved. In the case of the Milky Way (and the Solar System), a more compact configuration would have a numerically smaller gravitational potential energy. ...
• 111k
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### Why was more dark energy created during the creation of the Universe than normal energy?

It is not currently known why there is more dark matter than normal matter because no one knows what dark matter is yet. Dark matter is like normal matter in that it pulls space together with gravity. ...
Accepted

• 170k

### Dark energy vs. gravity

The density of Dark Energy is not very high. In a place with lots of matter, the attractive forces of gravity are greater than the repulsive forces of dark energy. In mostly matter empty space, the ...
• 629