30 votes
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 ...
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  • 1,790
28 votes

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 ...
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  • 65.6k
26 votes
Accepted

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 ...
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  • 95.3k
23 votes
Accepted

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 ...
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  • 10.4k
23 votes
Accepted

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 ...
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21 votes

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 ...
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  • 18k
20 votes

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 ...
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  • 34.3k
20 votes

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 ...
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19 votes
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 ...
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  • 7,737
19 votes
Accepted

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. ...
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  • 111k
18 votes
Accepted

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. ...
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15 votes
Accepted

How much dark energy will fit in an average cup of coffee?

Dark energy as expressed by the cosmological constant is, as the name implies, a constant of space. Therefore, in a cup of coffee, we get, for some static observer $t$, and a spacelike hypersurface $\...
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  • 14.4k
14 votes

Are modified theories of gravity credible?

Yes. Modified gravity theories are credible. Daniel Grumiller and Jerry Schirmer have pointed out some of the case against it, but there are deep, potentially intractable problems with a dark matter ...
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  • 3,683
14 votes
Accepted

How do scientists calculate the percentage of dark energy in the universe?

There are (at least) four ways in which the dark energy content of the universe influences things we can observe The cosmic microwave background is formed in the early universe when atoms (of ...
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  • 111k
14 votes
Accepted

Einstein and his so-called biggest blunder

I'll try and briefly run through some points (without mathematical detail) to see if this clears up any of your questions • You seem to have misunderstood the 'blunder' part: the 'blunder' wasn't ...
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  • 3,058
13 votes

Justification for new theories of Particle Physics and General Relativity

I am an experimental physicists, and the model in the first paper has not reached the level of experimental predictions, for LHC results. In fact except for the link you give the search at the CERN ...
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  • 222k
12 votes

Dark Energy / Accelerating universe: naive question

It's not as naive a question as you may think, and the answer is a lot more complicated than you may think. When we're calculating how the universe expands we assume it's isotropic and homogeneous (...
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12 votes
Accepted

Does dark energy annihilate energy?

Dark energy is an unknown or unattributed form of energy that is separate and distinct from the other forms of energy. It is not anti-engery. It is dark energy. Anti-energy (were such a thing to exist)...
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  • 24k
10 votes

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

I though I would discuss the transition from radiation to matter dominated phases and from there to the dark energy phase. A fair amount of this can be discussed with just Newtonian mechanics. General ...
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10 votes
Accepted

Why are we trying to fit GR in QFT when there's a chance that GR is incomplete?

Why are we trying to fit GR in QFT when there's a chance that GR is incomplete? There's not just a chance that GR is incomplete. It is certainly incomplete: it can't account for quantum phenomena. ...
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10 votes

If gravity is attractive, why doesn't the Milky Way contract?

If all the planets in our solar system, and all the stars in the Milky Way, were to all contract into a smaller volume, we would have a huge loss of energy and angular momentum. These are conserved ...
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  • 25.3k
10 votes

About the "worst prediction in all of physics"

What's confusing is that she claims this isn't a problem at all, because the cosmological constant has nothing to do with the vacuum energy from QFT. She says the cosmological constant is simply a ...
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9 votes
Accepted

Dark Energy / Accelerating universe: naive question

We indeed expect that the universe expands on the basis of the Big Bang theory. Hence by looking at higher redshift, i.e. further in time, you should expect objects to recede faster. This is reflected ...
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  • 3,708
9 votes
Accepted

Was the matter-energy content of our universe always distributed in the same ratios?

Short answer: The ratios have changed over time... drastically. This is a consequence of the expansion of our universe. Initially (and by that I mean after the conjectured inflationary epoch, which I ...
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  • 15.7k
9 votes
Accepted

What is the difference between the Big Bang Model and the Λ-CDM Model?

The Big Bang Theory is a much more general and less specific description of our theory about the origin of the Universe than the $\Lambda{\rm CDM}$ model (by the way, I don't think that the hyphen is ...
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9 votes

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

The short version: The amount of matter in the Universe is fixed, so as the Universe expands, matter density will drop because the same amount of matter will be spread out on more space. Dark Energy, ...
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  • 3,921
9 votes
Accepted

The Redshift of the CMB vs. Dark Energy

In terms of direct numbers at present, dark energy comprises about 70% of all of the energy in the universe. Radiation, on the other hand, makes up less than 0.005% of the energy in the universe. It's ...
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  • 24k
9 votes
Accepted

Does string theory accommodate effects of dark energy?

On one hand, according to observations, our universe has a positive cosmological constant $\Lambda>0$ associated with dark energy. On the other hand, string theorists have so far not rigorously ...
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  • 170k
8 votes

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 ...
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  • 629
8 votes
Accepted

If dark energy is accelerating the expansion of the universe, why can't I feel it?

Is it because the acceleration is too weak? It is too weak with respect to the four forces we measure. The fact that the four known forces are so much stronger means that agglomerates of particles, ...
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  • 222k

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