# Tag Info

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Dark Matter Your understanding of dark matter isn't bad, but here's a few clarifying details. Orbits: The speed of an object's orbit is related to 2 things: the radius of its orbit and the mass inside of it. In the solar system, over 99% of the mass is concentrated at the centre, so radius is the dominant effect on orbital speed. As we look at planets ...

18

Some additions to the answer of MBR: In fact, we do not know that dark matter and dark energy do exist, but we have indirect clues. You will often see claims that dark matter and dark energy are two of the major problems of cosmology today, including by professional astronomers, but this is an epistemological misconception: you cannot call a hypothesis a ...

10

Dark matter and dark energy are two different things, accounting for different observations. Dark matter: Dark matter is needed to explain, among other things, the rotation curve of galaxies. One could expect these rotation curves to decrease at large radii (because one should expect keplerian rotation for galaxies), and it is not the case, the rotation ...

9

This is an intriguing proposition, but I would ask how your hypothesis explains that the universe appears to be flat? That is with $\Omega_M + \Omega_\Lambda = 1$. The evidence for this comes from measurements of the cosmic microwave background, yet if we sum up all the matter (including dark matter), we only arrive at $\Omega_M \sim 0.3$. I do not think ...

8

The answer here is very similar to if you were asking about light. In principle gravitational waves might allow us to fractions of a second after the big bang. Electromagnetic waves can see back to where the cosmic background radiation formed, about 400,000 years after the big bang. You are right, the universe has expanded. At the present epoch it is ...

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Could dark energy (the mysterious accelerating expansion of the universe) be explained by "negative gravity"? But it already is "negative gravity". In general relativity, the stress-energy tensor $T_{\mu\nu}$ describes the energy, momentum, and stress of matter in spacetime. Through the Einstein field equation, it is connected with Ricci curvature $R_{\mu\... 8 The Hubble parameter is defined as the rate of change of the distance between two points in the universe, divided by the distance between those two points. The Hubble parameter is getting smaller because the denominator is getting bigger more quickly than the numerator. In the future, the cosmological constant,$\Lambda$could result in an exponential ... 8 No - the decreasing energy in the CMB is already well modeled in the Friedmann equations. The term in the density parameter that is proportional to$a^{-4}$is the contribution of radiation energy density to the evolution of the universe, the term proportional to$a^{-3}$is matter density (mostly dark, but includes ordinary matter),$a^{-2}$is the ... 7 The current supernova is a supernova of type Ia. Supernovae of type Ia are used as standard candles for distance estimates, especially used to determine the Hubble constant. Hence by a better calibration of this kind of supernovae, more about the reliability and accuracy of distance estimates can be learned. The expansion rate (in relation to the distance) ... 7 Disclaimer: I'm Dr. Kevin Croker, lead author on the ApJ series in question. I work on formal aspects of relativistic perturbation theory. I think the best way to answer your question is to just address all of the commenters' responses. I just made this SE account to respond to you, so I don't have sufficient reputation to reply as a comment yet. (For ... 6 AS the articles on the web suggest, Dark energy is the reason behind the expansion of universe. If some articles on the web suggest that, they are mistaken. Dark energy affects the acceleration of the cosmic expansion, but it is not necessary for the universe to be expanding, so it cannot be "the reason behind the expansion". How the dark energy density ... 6 My topological defect cosmology is a little rusty, but I'm pretty sure this is how it goes. Start with the fluid equation, $$\dot{\rho} + 3 {\dot{a} \over a} \left( \rho + p \right) = 0,$$ and the equation of state, $$p = w \rho.$$ Plug the equation of state into the fluid equation, assume a constant$w$, and you'll find $$\rho \propto a^{-3(1 + w)}.$$ ... 6 If a star 300,000 km away from you explodes, it'll normally take the radiation from that explosion one second to start vaporizing you. If the space between you and that exploding star inflates to 2.6X10^23km (28 billion ly) one half second after the explosion, the radiation will take 14 billion years, plus a half second, to reach you, and won't vaporize you. ... 6 Is Cosmos seriously using that exact number? Egads... if they are, don't take it too seriously, but otherwise they're probably conceptually correct. How do we know it was dark energy? In cosmology, the ΛCDM model fits numerous observations, most notably those observed by the WMAP satellite, but also others, to the Friedmann-Robertson-Walker family of ... 6 What's outside the observable Universe, we can't say anything about, but averaged over large enough scales ($\gtrsim$a billion lightyears), it does indeed seem to be expanding uniformly. However, the presence of mass, or more generally energy, retards the expansion. This means that on the scale of clusters of galaxies, the Universe expands more slowly, and ... 6 A cosmological constant should be considered a special case of dark energy. The effective stress-energy tensor for a cosmological constant is proportional to the metric$g_{\mu\nu}$, so in a local inertial frame will be proportional$\mathrm{diag}(-1,+1,+1,+1)$. This is equivalent to perfect fluid with energy density and pressure directly opposite one ... 6$H_0^{-1}$is only a rough estimate for the age of the universe and you have correctly identified the reasons why not. A correct age estimation relies on knowing$H_0$and the densities of matter and dark energy, so that the past expansion history of the universe can be correctly modelled. Even this relies on an assumption about how dark energy behaves. A ... 6 First of all, if there were no dark matter (DM), you wouldn't ask this question, since structures — including galaxies, stars, planets, and you — wouldn't have had the time to form in the early Universe before it had expanded too much for gravitational collapse to occur. But let's use magic and make the galaxies anyway: The CMB (specifically the power ... 6 There are several reasons dark energy cannot be pressure due to the Pauli exclusion principle. First of all, pressure does not cause expansion of the universe, because pressure is not a force-- pressure gradients are a force (per unit volume), and the cosmological principle precludes them. Indeed, the pressure that exists everywhere only appears as a force ... 6 You can probably get most if not all of your questions answered by perusing the main DESI web site, which I encourage you to check out. There is, for example, a nice video describing the assembly of the main focal plane elements (the fibers and the associated robot positioners) here. But in simple terms: the circular focal plane is divided into ten wedges (... 6 That will never happen. We will always be able to see the galaxies in the Local Group. We won't get separated from them by the expansion of space because the group is bound together gravitationally. But the rest of the galaxies will be beyond the cosmic light horizon in 150 billion years or so. However, because of the mutual gravitational attraction it's ... 6 The brackets refer to the average, so$\left< x^2 \right>^{1/2}$is the root-mean-square (RMS) of$x$. That is the square root of the mean (or average) of the square of multiple$x$s. The RMS average is useful when a quantity can be either negative or positive. For instance, a sine or cosine wave has an average of zero over one cycle, but its RMS ... 5 There are two problems that arise here. One obvious problem is that the Casimir effect is attractive1, while dark energy is repulsive. The other problem is one of scale. Casimir (1948) shows that, between two particles (instead of the oft-cited case of two plates) $$\delta E \propto R^{-7}$$ That's an enormous drop-off. On large scales, this should be ... 5 If dark energy varied by location, then plots of 1a supernova brightness vs redshift should vary depending on which direction you look in the sky. AFAIK, that's not the case. For example, although coords are not accounted for, there's not a lot of scatter in this plot: In the relationship between the distance and redshift of Type 1a supernovae, the data (... 5 There is in fact a cosmic gravitational wave background. These waves are expected to be stochastic, having originated in the early universe (much earlier than the cosmic microwave background). Random fluctuations were subsequently stretched during inflation, making them observable over many wavelengths. A good and reasonably up-to-date introduction I read is ... 5 Not very strong at all. I get a rough figure of$3.725\times 10^{-9} m/s^2$. To perform that calculation I made a few simplifying assumptions. Assume that we can ignore everything outside the observable universe. Assume that the observable universe is a static homogeneous sphere with a radius of 46.6 billion light-years, and a mean density of$\rho = 0....

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Dark matter can, and probably does, fall into black holes and affects them just as any other form of matter or energy falling into them does. It doesn't fall in a huge amount because, like anything else approaching a black hole it usually isn't aimed straight in, but rather misses a bit and swings by and escapes again. Unlike normal matter, dark matter ...

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Supplemental to @PeterErwin's answer, some more details on the five thousand "robots". Each fiber has a circular "patrol area" with a diameter of 12 millimeters, and these are located on a hexagonal array with a pitch (nearest neighbor distance) of 10.3 millimeters. Motion is implemented with eccentric axis (Θ–Φ) kinematics. Instead of x-y or r-Θ which use ...

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Another thing about dark energy/matter: People have a pretty good idea that dark energy exist because, when you chart the expansion of two objects in the universe over time, from its origin, there is a bell curve. Basically, the speed of the universe's expansion started out faster, than slowed down, and recently (or at least relatively), the expansion has ...

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Yes and no. First, the answers here should give you some understanding of dark energy and its effects. The net effect of dark energy is tiny over small distances. If it is given by a cosmological constant, then its effects on gravitationally bound systems is independent of time (hence 'constant'). Observational evidence puts us at least close to the ...

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