# Tag Info

16

Excellent question! In short, there are two logical possibilities to explain the data: There is dark matter and a cosmological constant (standard model) Gravity needs to be modified Interestingly, both possibilities have historical precedent: The discovery of Neptune (by Johann Gottfried Galle and Heinrich Louis d’Arrest) one year after its ...

11

Dark Energy was discovered in 1998 when two separate teams of astronomers (see here and here) studied Type Ia supernovae. The use of Type Ia supernovae is particularly important because they have a very specific luminosity as a result of how they occur. Both groups independently found that the distances to the galaxies which hosted the supernovae they were ...

9

While it is possible that gravity still needs to be modified, it is looking increasingly unlikely that there ISN'T some form of dark matter. In particular, the observation of the bullet cluster is a tall order for the various modified gravity theories (though, arguably, the extra fields in something like bimetric gravity or TeVeS could be self-coupling in a ...

7

Nope. Gravitational radiation is a kind of radiation and it has a completely different equation of state than the cosmological constant. The cosmological constant has pressure equal to the energy density with a minus sign, $p=-\rho$: the stress-energy tensor is proportional to the metric tensor so the spatial and temporal diagonal components only differ by ...

7

What we directly observe is that the Universe was expanding and the expansion was accelerating during the recent five billions of years or so (it was actually not accelerating before that because the dark energy wasn't dominating). The prediction that it will continue to expand and accelerate results from a "clever scientific extrapolation" – from writing ...

7

In the late 1990's, astronomers were measuring the speed and distance of distant galaxies away from us, and trying to see how fast the universe was decelerating. A surprising find was made, that the expansion of the universe is accelerating, seeming to defy the laws of gravity. Study after study has been made to confirm this, and everything points to it ...

6

Time invariant dark energy is the simplest form. Time invariant dark energy appears in the GR field equations in the same way as a cosmological constant, making it easy to handle. There are plenty of other suggested forms of dark energy that do vary with time. See for example wikipedia.org/wiki/Quintessence_(physics).

6

Actually, that first statement is not correct. The universe isn't expanding due to dark energy. It's accelerating due to dark energy. The normal expansion, called metric expansion, is an effect of general relativity. When you get a homogeneous distribution of matter or radiation(a perfect fluid, a uniform gas, radiation, a homogeneous distribution of ...

6

First of all, dark matter and dark energy, despite their naming, are two very different concepts. We don't really have any good reason to group them together, other than the fact that both represent things we don't understand. Thus they are not necessarily backed by the same sets of evidence. Why we believe these things exist As it happens though, some ...

6

Be careful when trying to intuit how sensitive the integral formulation is to changes in $w$. The equation of state parameter only enters as part of the exponent of $1+z$, so for $z \approx 0$, $w$ has approximately no effect: $1^0 \approx 1^\epsilon$. To illustrate with equations, suppose you already know $\Omega_\mathrm{M}$ and $\Omega_\mathrm{DE}$ ...

5

You have to be careful to distinguish between curvature of space and curvature of spacetime. When we say that the Universe is flat on large scales, we're talking about space -- that is, about a slice through spacetime at constant cosmic time. With respect to spatial curvature, statement 1 is correct: we do have zero curvature on average, and positive ...

5

Actually, on a dark night, the fraction of the sky that is light is pretty negligible. That's what it means to be a dark night ;-) It's actually not hard to get an estimate of the density of light in the universe. Let's say that "light" includes photons of all wavelengths (not just visible light) for simplicity. A straightforward way to do it is to point a ...

5

The total energy in the space does increase, precisely because of the reason you mention. Energy is not expected to be conserved, because the metric is not invariant under time translations. What does hold is the first law of thermodynamics, $dU = -P dV + \cdots$. Since the pressure in this system is negative, this is one way of seeing the origin of the ...

5

The question Qmechanic linked to is a duplicate, but I think the answer to that question is wrong. Well, actually it's only technically wrong since there is no practical way to extract useful energy from dark energy, but in principle it could be done. If you tie two spheres to each end of a rope and leave them floating in space then the expansion of space ...

5

You can fit a value for Dark Energy (DE) using near or far supernovae (SNe), the benefit of distant SNe is purely from a statistics perspective: you want to minimize your errors on your best fit value of DE ($\Omega_{DE}$). To determine the uncertainty in your fit ($\sigma_{\Omega_{DE}}$) you have to do error propagation which will show a dependence on the ...

4

Dark energy, dark matter, and gravity are intimately entwined concepts, certainly. The question seems to blur dark matter and dark energy together or use the terms interchangeably, but they are separate theories attempting to reconcile separate discrepancies. One facet contributing to the majesty of Einstein's Equations for gravity (see ...

4

Good question. I have a vague idea about how errors like this are catered for so I'll take a shot at answering your question. I stand to be corrected by anyone who's closer to SNe Ia cosmology. The short answer is that the discovery of dark energy is based on empirical calibrations, so any scatter in the progenitors of the supernovae is already accommodated. ...

4

Yes, dark matter is really matter. Specifically, it seems that dark matter consist of massive particles that "clump" around galaxies. This can be seen via gravitational lensing (and other techniques) which allow one to form a map of the gravitational being produced in some region. Once one subtracts of the gravitational field of all the non-dark matter ...

4

Dark Energy is supposed to substantiate the theory of cosmic inflation; the substance itself is a hypothetical one. There is also a little controversy, though: Albert Einstein provided the idea with his cosmological constant but without evidence it fell by the wayside - the result being its exclusion from certain fields of physics, while still being useful ...

4

No, there isn't much beyond the acceleration parameter of the universe to support DE. In fact, if you're willing to abandon homogenity and isotropy, you can even get away without DE by choosing a void model, where you replace a fine tuning of the matter distribution with a fine tuning of the dependence of the density on radius from the 'center of the ...

4

I think the strongest evidence comes from the CMB fluctuations, namely the location of the first acoustic peak. This gives the overall geometry of the Universe ($\Omega_{tot}=1$; the Universe is flat). Then with a multitude of observations of dark matter (e.g., galaxy cluster counts, large-scale structure, and weak lensing) to get $\Omega_{matter}=0.3$, we ...

4

I found it surprisingly hard to find an authoritative statement of the density of the CMB. According to this article it's about $5 \times 10^{-34}\mathrm{g\ cm}^{-3}$, and since the critical density is somewhere in the range $10^{-30}$ to $10^{-29}\mathrm{g\ cm}^{-3}$ photons don't make a significant contribution. Photons wouldn't be dark of course. If ...

4

There are a bunch of misconceptions in your question: 1) General relativity is a field theory, first and foremost. While we can certainly talk about energies and such in this contect, it is not the most natural language for the theory, which is phrased in terms of the fields themselves--where is the potential energy in an electromagnetic field ...

3

If you are asking whether the proposition that "dark energy and dark matter are necessary to explain the experimental astrophysical observations within the framework of known physics" has been found false, the answer is no. If you are asking whether it is possible to falsify the proposition, i.e. if new data could show that the proposition is false, this is ...

3

This has been proposed as a alternative explanation of the accelerated expansion of the universe. Note that this implies that the universe is non-homogenous on the largest scales and that we happen to be near the center of a relatively lower density region of the universe and that we are surrounded by higher density regions. So it requires that we are in a ...

3

This cannot explain dark energy, even in principle, for the following reason. If dark energy were any kind of "invisible light" (that is, electromagnetic radiation of very large wavelength), its pressure and energy density would be related in the standard way for radiation: $p=1/3 \rho c^2$, where $p$ is pressure, $\rho$ is energy density, and $c$ is the ...

3

I always look up when perpetual motion type machines come up, i.e. more energy out than in. I also asked a question here on the possibility of milking energy out of the vacuum. It is always possible that by serendipity somebody may hit the pot of gold of energy, never mind how he/she interprets it. If a machine gives more energy output than input, the theory ...

3

Hmm, this looks awfully like a homework question. Giving you the benefit of the doubt, the answer is (B). The universe is (to a good approximation) described by the FLRW metric. Dark energy causes the cosmological constant term to be greater than zero. This is responsible for the accelerated expansion. A possible confusion is that (A) is probably caused by ...

Only top voted, non community-wiki answers of a minimum length are eligible