# Tag Info

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There are two separate questions there. The easiest one to answer is how we measure the vleocity of the Earth, Milky Way etc, because we measure it relative to the cosmic microwave background (or CMB). If you measure the CMB in all directions and find it's the same in all directions then you are stationary in comoving coordinates. However if you find the ...

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The low-entropy initial state of the universe is an open problem without a satisfactory answer. Your question is the first time I've heard the suggestion that the initial state should have been a crystal; you remind me that the quark-gluon plasma, which was the state of the universe while it was too hot for nucleons to be stable, has been shown to be a ...

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I assume your question was asked with the implicit "and everything else is kept the same" (still GR + standard model, just with one parameter tweaked). This would have a large effect, because now the neutron would be much more stable! The neutron is already quite stable (~ 10 minute half life), due to the tight energy constraints in the reaction decaying ...

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The most compelling evidence of GR in presence of matter is, in my opinion, in neutron stars. These objects have a surface gravity $SG$ that is (geometric units): $SG_{NS}=GM/c^2 R \simeq 0.1$ This value is telling us that we can't use Newtonian gravity because we are in the strong field limit. For comparison, the sun has $SG_{SUN}=GM/c^2 R \simeq 10^{-5} ... 9 If our ideas about cosmic evolution are correct, galaxies that are visible today will in principle remain visible in the future. As time goes on, light from more and more distant galaxies will be able to reach us, and the number of observable galaxies will increase. However, there exists a cosmic event horizon$^1$, so this is an asymptotic growth: There's a ... 8 There are a number of different frames of references. For the velocities of celestial objects we use: (i) The geocentric frame: This is a velocity measured with respect to the Earth's centre. Obviously this is quite useful for artificial satellites, but also for things like meteors. (ii) The heliocentric frame: this is the velocity as seen from the centre ... 8 the two paradigmatic cases that illustrate these two possibilities is a gas, for the first, and a crystal for the second. Paradigms and examples are well and good, but be careful not to assume they are the only possibilities. In particular, black holes have entropy -- a lot of entropy. In fact they saturate the Beckenstein Bound. The entropy of a black ... 6 OK, I found a recent link: Planck versus BICEP2 Despite the new data, the collaboration did not give any insights into the recent controversy surrounding the possible detection of primordial "B-mode" polarization of the CMB by astronomers working on the BICEP2 telescope. If verified, the BICEP2 observation would be "smoking-gun" evidence for the rapid ... 6 Frame-dragging effects are dependent on the spin of the central object, have been measured by experiments such as Gravity Probe B, and are definitely not dependent on the central metric. Also, any effects on a galactic scale are best quantified in terms of a continuous matter distribution, since the central black hole is a small fraction of the galaxy's ... 5 Energy conservation stems from Noether's theorem applied to time (i.e., time-invariance leads to energy conservation, similarly to how spatial-invariance leads to momentum conservation). Since the universe is expanding (and accelerating at that), the state of the universe today is different than it was yesterday and will be tomorrow, hence energy ... 5 If they are much far away will they have relative velocity of separation greater than speed of light and if so how can we even detect such galaxies. We can't detect such galaxies. Redshift goes to infinity at the cosmic horizon, and we cannot see beyond. Note that the cosmic horizon is different from the Hubble sphere: At the former, relative velocities ... 5 Even in special relativity, the claim that light always travels at a constant speed of c is only true in inertial reference frames, in non-inertial coordinate systems like Rindler coordinates, the coordinate velocity (change in position coordinate with respect to coordinate time) may vary. In general relativity, all coordinate systems covering large regions ... 4 What I will state is speculative and based on the statistical mechanics derivation of entropy, and just the way I view it and do not consider that there exists a problem. After all thermodynamic theory emerges from the underlying statistical level of atomic and molecular interactions. where p_i is the probabability of microstate i. Setting aside quantum ... 4 No, throwing matter out isn't contrary to a BH; there is often an accretion disc surrounding the black hole and that is what forms the jet. No, the ejected material cannot condense to form a galaxy. A galaxy requires the material to be gravitationally bound to some central point, a jet moving at$\sim c$is moving too fast to be gravitationally bound to ... 4 Your idea that a quantum fluctuation created the universe is a misinterpretation of the suggestions that I have heard. Explaining why requires introducing a few ideas, so bear with me while I do this. The object we think of as the universe is made up of two bits: a manifold equipped with a metric = spacetime some matter/energy The manifold and metric ... 4 Correction of typos and clarifications In the Friedmann equation, '$\rho$' is strictly speaking$\rho_m$, the mass density. Hence the presented Friedman equation has to be changed as follows: $$H = \sqrt{\frac{8\pi}{3}G\rho_m}=\sqrt{\frac{8\pi}{3}\frac{G\rho}{c^2}},$$ in order to use the expression given below for the energy ... 4 Javier, brings up some interesting points. However, protons and neutrons get most their mass from relativistic quarks. If the quarks could be slowed down they would weigh a few electron masses. So what ever is responsible for giving the electrons mass its value seems to be giving the quarks their rest mass. I just checked the up and down,they are around 4 ... 3 There are scientists that do not accept the Big Bang (and more). That doesn't mean that there isn't evidence for the Big Bang (there is), nor does it imply it may be wrong. All we can say is that some scientists consider the Big Bang to be a fact because the evidence is overwhelming, some who think it is the best cosmological model because there is much ... 3 As far as I know, "a" Universe is caracterised by fundemental constants such as the speed of light, Newton's gravitationnal constant, the Planck constant and so on. You could distinguish between two different universe from the variation of these values I think. 3 If we define a scale factor$a$for the universe (could be the distance between two galaxies), then this scale factor will change with time. This is also true in flat or infinite universes, so long as the Hubble parameter is$>0$(i.e. the universe is expanding). The energy density contained in the cosmic microwave background will scale as the energy of ... 3 Special Relativity states that as an object increases in velocity, it gets more dense (mass increases, volume shrinks). No, no, no. Absolutely not. But I know why you think this. Back many decades ago, before I was born, lots of physicists wrote physics papers and textbooks without really understanding relativity. They made mistakes. Not necessarily ... 3 From a philosophical standpoint, it is impossible for me not to exist. I'm not sure about you, but I know that I am a thinking, cognizant mind perceiving some sort of reality y having an experience. Whether that experience corresponds to ultimate reality is irrelevant. Since we've yet to solve the problem of hard solipsism (with no known way of even how to ... 3 In this paper by Gott et al., on p. 466 they define the "future visibility limit", saying in the published version that "No matter how long we wait, we will not be able to see farther than this", and on p. 7 of the arxiv preprint they similarly say "If we wait until the infinite future we will eventually be able to see the Big Bang at the co-moving future ... 3 Such galaxies cannot be detected. and Quantum entanglement is just a correlation between two bits of information and it no way does, and no way can be used to transfer information faster than the speed of light. Any light sent from such galaxies towards ours would show a wavelength of infinity after considering Doppler shift. In short these galaxies can ... 3 There are basically two measurements which take place here: (i) the redshift of the supernova (ii) the peak brightness of the supernova (though this latter is a bit more complicated than that). The distance is not found from the redshift, it found from the apparent brightness of the supernovae. The supernovae in question are Type Ia and act as "standard ... 3 The first thing I want to introduce is the standard candle, which is an astronomical object with a very well known luminosity and emission profile. One of the best standard candles we have is the Type 1a Supernova. This type of supernova only occurs in a very specific way such that all type 1a supernovae have approximately the same luminosity and emission ... 2 These galaxies are not out of reach. The Hubble Sphere is the volume of space surrounding an observer where everything inside the sphere moves away from the observer with a speed less than c. Following this logic it immediately follows that the Hubble Sphere is equivalent to the cosmological event horizon that @Hritik is discussing in his answer. ... 2 In relativity there is no absolute speed because there is no notion of absolute space or time--your speed can only be measured relative to some reference frame (a coordinate system which assigns a position coordinate to each object at each time coordinate), usually an inertial frame (the speed of a light ray is the same regardless of which inertial frame you ... 2 A recent paper by Dan Whalen titled, Finding the First Cosmic Explosions. I. Pair-instability Supernovae discusses this very problem. The pair-instability supernova (PISNe) is a special case of massive stars, around 100$M_\odot\$, in which the thermal pressure inside the star is reduced via the production of electron-positron pairs. Runaway thermonuclear ...

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Speed is a distance (separation between two well defined points in space) traveled over a time. The speed of Earth you quote is the orbital velocity. We know how far away the Sun is and we know the shape of the orbit, so we know how far the Earth travels relative to the Sun (distance) per year (time). Likewise the speed of the Milky Way is also given ...

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