# Tag Info

18

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 ...

10

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 ...

8

The cosmologically relevant light is the cosmic microwave background (CMB), not radiation from stars. The energy density of the CMB is about $10^{-13}$ J/m3. This is of the same order of magnitude as the energy density of starlight within our galaxy, but most of the universe is intergalactic space where the density of starlight is much lower. The average ...

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

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 ...

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

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 ...

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

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 seem to be caught up on the word "dark." The reasons both things are called "dark something" represents our incomplete knowledge. Beyond this, dark matter and dark energy are no more related than Superman is related to superconductivy, or lightbulbs are related to light exercise. Dark matter is the term for what appears to be gravitating mass spread out ...

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 ...

5

It's not clear if you're asking for details of how the CMB power spectrum is analysed, or whether it's a general question about how this sort of measurement is made. I'll assume the latter - hopefully this will be of interest to others even if it isn't what you intended. There is a good discussion of the procedure in the Wikipedia article on the Lambda-CDM ...

5

First of all, the AdS space is a hyperboloid-like maximally symmetric space so it doesn't ever "collapse". In general, a negative cosmological constant may speed up the collapse into a Big Crunch but what exactly happens depends on the distribution of matter and/or initial boundary conditions, too. The Universe around us isn't an AdS space; it has a ...

5

They are proportional so essentially the same, but $\Omega_\Lambda$ is a convenient dimensionless number. Straight out of Weinberg's newer cosmology book: $$\Lambda = 8 \pi G \rho_V,$$ where $\rho_V$ is the vacuum energy density, and $$\rho_{V0} = \frac{3 H_0^2 \Omega_\Lambda}{8\pi G}.$$ Putting them together $\Lambda = 3 H_0^2 \Omega_{\Lambda0}$. ...

5

Well... the mass of the sun is $2 \times 10^{30} kg$. If it loses $4 \times 10^9 kg$ per second, it would take 160 billion years for it to lose 1% of its mass. The dark matter content of the universe is theorized to be 26.8%. So, the total mass contribution from photons cannot possibly account for the missing dark matter. Also, if light from stars really ...

5

You do not state your age or your educational background in your profile , so I will assume that you only know physics at the highschool level and particularly gravitation from videos and simplified explanations. 1) It is wrong to think that matter disappears like bubbles blown out on the water. The grand majority of elementary particles are bound in stable ...

5

Renormalisation is a computational technique. Calculating scattering amplitudes directly gives infinite results, but the process of (i) regularising the theory (ii) calculating using the regulated theory then (iii) taking the regularisation parameter to its physical limit gives the finite result that matches experiment. By contrast the computations in GR ...

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

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 ...

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. ...

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