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34

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 ...


14

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 ...


12

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 ...


12

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 getting smaller all the time. Indeed, we'd all be thrilled if, as measurements got more precise, $w \ne -1$ turns out to be the case because the $\Lambda$CDM ...


12

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 document server gives nothing, and the word "weakton" does not yield discussions or appraisals. So the experimental physics community is overlooking this, even ...


12

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) would annihilate any form of energy. Dark energy is called "dark" because we aren't exactly sure what it really is or what causes it. The most abundant forms ...


11

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 hydrogen) first formed and the universe became transparent to the radiation that was within it. There are small fluctuations in the CMB which reflect small ...


10

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 ...


9

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 ...


9

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 (this just means on average it's the same everywhere) and it has a scale factor that is normally written as $a(t)$. The scale factor tells us how much the ...


9

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 ...


8

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 in the linear relationshift that first measured by Hubble $$v=H_0D, $$ where $v$ is the recession speed, $D$ the distance to the object and $H_0$ the Hubble ...


8

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 ...


8

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 ...


8

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).


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

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 ...


7

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 ...


7

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 ...


7

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 ...


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 $\Omega_\mathrm{DE}$ ...


6

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 $\Lambda = 3 H_0^2 \Omega_{\Lambda0}$. ...


6

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 ...


6

A positive cosmological constant leads to positive scalar curvature by definition. Just trace over the Einstein equation and you end up with $$ R = 4\Lambda - 8\pi T $$ which is just $$ R = 4\Lambda > 0 $$ in vacuum. The implicit, but more interesting questions are probably the following ones: Why can we interpret the cosmological constant as dark ...


6

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, up to the scale of galaxies are not internally affected, they keep their structure intact, like the famous raisins in the rising bread. It is only at the ...


6

In Newtonian mechanics, we have potential energy and kinetic energy. All types of energy can be classified as one or the other -- never both or neither. But when you deal with fields, this distinction doesn't really work. For example, a light wave (electromagnetic field) carries energy, and this energy doesn't fit neatly into either the PE or the KE ...


6

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 ...


6

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 ...


6

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 ...



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