# Tag Info

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No. Creation implies causality. Causality implies the conceptual factor of "time". Time is a derivative of space, hence space-time. Space contains all things, and is infinite.

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If we were to interpret the cosmological constant as being geometrical (i.e. put it on the LHS of the equation), how would it be described? Dark energy is said to be responsible for the increasing expansion of the universe, so let set that aside. Let's focus on the cosmological constant, which is "the value of the energy density of the vacuum of space". ...

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The universe needs to be near zero energy to not crumple in on itself. Who told you that? A gravitational field is a place where the motion of light through space is curved and where objects fall down. Because space is inhomogeneous, this being modelled as curved spacetime. It isn't a place where space is falling towards the centre. The universe didn't ...

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Cosmological redshift can be interpreted as the accumulated Doppler redshifts between infinitely many frames of reference, each locally at rest and moving away from us at a certain velocity. So you cannot distinguish the two because they are basically different cases of the same thing. However, as it says in @Rob Jeffries' answer above: when looking at ...

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I think there are duplicates of this, but couldn't immediately locate them. The answer is you cannot tell observationally whether a single redshift measurement is caused by the expansion of the universe or by something moving away rapidly. However, if one wished to interpret the ensemble of redshifts that we see in a non-expanding universe, then you must ...

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Today, most physicist "believe" in the inflationary cosmology which solve many problems in cosmology and it doesn't indicate our universe started with a singularity.

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The latest constraint on the curvature of the universe comes from the Planck Mission which indicates that the universe is very flat today.

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The following graph indicates the amount of radiation, matter (which almost is entirely made of dark matter), and dark energy vs time. We are at a point in time where dark energy has just started to dominate. For instance, if you go back to the first few years, radiation was the most abundant source of energy.

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First, the early universe was incredibly hot. Second, Gravity is the weakest force. Third, when we talk about the early universe, we analyse things up to the point that gravity is not important. Why? Because we don't have a quantum theory of gravity. Forth, you need to think of the early universe as an infinite dense soup of particles. It is homogenous and ...

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First of all the current expansion rate of the universe is called the Hubble constant and the best current value for the Hubble constant is: $67.80\pm 0.77\ km/s$ per megaparsec - (from the Plank satellite data). Notice the units are a speed divided by a distance which means that the units are really $(1/time)$ and in inverse time units the value would ...

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The little $h$ is a historical artifact, one that will probably die out soon enough. The thing is, $H_0$ was extremely difficult to measure precisely for many decades after its importance was realized. At some point, cosmologists were divided between the "$H_0 = 50\ \mathrm{km/s/Mpc}$" and the "$H_0 = 100\ \mathrm{km/s/Mpc}$" camps. Because the quantity ...

-1

If the universe expands by a factor k, does that mean that the distances within the universe also expand It means the distances between the galaxies expands. Check out the raisin-cake analogy. the time light needs to travel a given distance increases That depends on the speed of light. See the Wikipedia variable speed of light article. Physicists such as ...

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For some function $f$ of $x$, the logarithmic derivative is simply $$\frac{\mathrm{d}\log f}{\mathrm{d}\log x} = \frac{x}{f} \frac{\mathrm{d}f}{\mathrm{d}x}.$$ You can check that this follows from the chain rule applied to $\mathrm{d}g/\mathrm{d}y$, where $g = \log f$ and $y = \log x$. This is a common thing to see in astrophysics, since if we have a power ...

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Usually, when I encounter infinity in my classes or in my work, I define infinity relative to something--that is, it's usually something that is very, very large that, for all intensive purposes, is the usual mathematical notion of infinity. Empirically, I don't think we have ever come to witness true infinity. Of course, some theories predict infinities. ...

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Assuming you found a way and managed to accelerate above light speed without disintegrating, and went to the edge of the universe... I'm confident you can't go faster than light, but when it comes to the edge of the universe, I'm also confident that nobody knows any answers. However people say they do and state categorically that there is no edge. For ...

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Light speed travel is impossible, so you are asking what happens to a system when we totally ignore the system. Aside from that, the universe probably does not have an edge. The observable universe does have an edge, but you can never reach it. This is because when you move, the the edge of your observable universe moves with you.

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Do clocks measure conformal time? No. I would venture to say that clocks don't literally measure time at all. A clock isn't some cosmic gas meter with time flowing through it. What clocks do is "clock up" some kind of regular cyclical local motion and show you a cumulative result that you call the time. A pendulum clock clocks up the swings of a pendulum. A ...

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Firstly, notice that the Frequency of light used to measure time will not remain constant. I wont use this but do take note. Secondly notice that If the object is rigid, then and One end is fixed at x = 0, then the other end in the co-ordinate systems used will be at $\Delta x = \frac{\Delta L}{a(t)}$ So Now light using equation 1 from your question we ...

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What is the exact meaning of homogeneity in cosmology? Conifold and Milad have adequately explained the distinction between homogeneous and isotropic, so I'll answer on a different tack: See the Einstein digital papers where he said 'empty' space in its physical relation is neither homogeneous nor isotropic. A gravitational field is a place where space is ...

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Here is a short summary inspired by Barbara Ryden: Homogeneity: No preferred location Isotropic: No preferred direction And here are some examples to clarify things: Example of homogeneous but not isotropic: A forest, it looks the same no matter where you are, but trees make the vertical direction distinct. Example of Isotropic but not homogeneous: When ...

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Homogeneity in cosmology means uniformity from point to point, not only in composition or content, but in geometry as well. An empty space with a singularity is still non-homogeneous. Isotropy at every point does imply homogeneity, but we are not in a position to observe the universe from every point. Mathematically, isotropy at any two distinct points ...

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I found an article by E. J. Copeland, D. J. Mulryne, N. J. Nunes, M. Shaeri that explains this, it's called Super-inflation in Loop Quantum Cosmology. This is part of the answer I wrote most the equations come from this article unless I cite otherwise. According to Loop Quantum Cosmology, in super inflation a smaller number of e-folds are required. This is ...

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You set $\rho$ equal to one for no reason. In detail the expression for $\Gamma$ is: $$\Gamma^1_{01}=\frac{1}{2}\sum_\rho g^{1\rho}\left[\frac{\partial g_{1\rho}}{\partial x^0} + \frac{\partial g_{\rho 0}}{\partial x^1} - \frac{\partial g_{01}}{\partial x^\rho}\right].$$ And since $g_{01}$ equals zero (since your metric is diagonal), all four partial ...

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See D. Hogg's Distance measures in cosmology, 2000 http://arxiv.org/abs/astro-ph/9905116 Section 7, Luminosity Distance, p. 6 $D_L=(1+z)^2 D_A$ follows because the surface brightness of a receding object is reduced by a factor $(1+z)^{−4}$, and the angular area goes down as $D^{-2}_A$.

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Red shift. is a really quick answer. whichever way we look, stuff is moving away from us. and the further away it is the faster it's moving.

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This relation is quite important, non trivial, and mathematical, and was proved by Etherington along with the other closely related theorem in this paper I. M. H. Etherington (Philosophical Magazine ser. 7, vol. 15, 761 (1933)) This theorem only depends on photon conservation and the fact that photons only travel in null geodesics in Reimannian ...

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Yes, the vacuum energy of a spacetime lattice with finite spacing and periodic boundary conditions within a box of finite size is finite. One would not call this "quantizing", though, rather discretizing because we are not carrying out any "quantization procedure" in the sense of going from a classical to a quantum system. In this approach, the finite size ...

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In the FLRW model which is used today normal (baryonic) matter and dark matter together add up to matter. It dilutes with the growing radius to the third power (because volume is proportional to radius³). So the ratio matter : dark matter is and was always the same, at least in the plot you showed and the model that was used there. Edit: Maybe the 5.25 to ...

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The ratio of dark to baryonic matter is 5.25 in the first diagram and 5 in the second diagram, but I don't think the difference is significant. We don't know the densities with absolute certainty, especially near the Big Bang, and the small difference between the ratios is probably just down to the uncertainties in the densities. We would expect the ratio ...

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Here's the horizon problem: Look at the sky. Look at one side of the sky. Then look at the other side of the sky. The light from one side has just now reached you, as has the light from the other side. When we look back to the earliest observable moments of the universe, the Cosmic Microwave Background (CMB), we do the same thing. We look at light that was ...

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The phenomenon you are referring to is Poincaré recurrence. The idea is that if a closed system has only a finite number of possible states then it must eventually return to a state that is has been in before. However the universe is not a closed system with a finite number of states and the recurrence theorem does not apply to it. For example the average ...

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Using the standard model of cosmology we calculate the Hubble time to obtain an estimate of the age of the universe. Yes, 13.8 billion years. But IMHO there's an issue worth discussing, to do with something John said in another answer: "A distant observer sees falling objects slow as they approach the event horizon and asymptotically approach zero speed at ...

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how the hell does relativity justify that the character can be thrown in black hole and survive ? This is a big question and scientists (physicists) are asking the question that "what will be the fate of an astronaut falling into a black hole?" ,since the concept of general relativistic black holes have arisen. The answer is the astronaut may not ...

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Strictly speaking the FLRW metric doesn't specify that time starts at the Big Bang. It specifies only that the Big Bang is a singular point so it is impossible to analytically continue a geodesic back in time past the Big Bang. If it helps to make things clearer, exactly the same happens with an object falling into a black hole. A geodesic that crosses the ...

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It means, that the cosmological perturbations satisfy Gaussian initial conditions. This means, that the probability of the perturbation amplitude has a Gaussian shape about the mean value. Considering linear perturbation theory, the initial Gaussian probability distribution will remain Gaussian for all times, e.g. today. It means, that there is e.g. no ...

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Your equation applies to a perfect fluid that has no viscosity. N-body simulations are for dark matter only and the dark matter is generally assumed to only interact gravitationally. There are hydrodynamical simulations that include baryons but those are more computationally intensive and so generally done for smaller simulations. If you are just interested ...

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There's no "inside" a black hole. The closest thing a black hole has is a horizon. And you can actually get out a part of what falls below the horizon - just not more than 50% (in theory). I'm not sure about the "transverse" part, that's not a term I've encountered in relation to black holes. Are you sure you got the spelling right? There are multiple ...

-1

I agree with what BowlOfRed said, but I'm going to give an answer with a different nuance. So why is the night-sky dark rather than uniformly painted at the brightness of an average star? Because the universe isn't infinite. Big bang cosmology describes a universe that started small some 13.8 billion years ago and has been expanding ever since. It's been ...

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Olbers’ Paradox says that in an infinite universe every line of sight will end on a star. That statement is incomplete. The paradox requires not only an infinite universe, but also one that is both static and infinitely old. Neither of the second two statements are true for our universe. Your question considers the effect of aging. As our position ...

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The particle horizon is the distance from which light emitted at the moment of the Big Bang will just now be reaching us. The CMB was emitted 380,000 years after the Big Bang. So the CMB radiation we see has been travelling for less time than the light emitted at the Big Bang, and therefore the CMB radiation has travelled a shorter distance than the ...

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The concept of the expansion of the universe is hard to get your head around. We describe the universe is infinitely large, and it is expending into itself. So, there is no outside, You can't leave the cosmos. If you could find the edge of the universe and exit through that edge, you would re-enter the universe from the other side. Edges of the universe are ...

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1) What you call $\rho$ should really be called $\epsilon$ (this is the energy density, not the particle density). 2) The thermodynamic variables $\epsilon$ and $P$ are expectation values of certain operators in a thermal ensemble. You should not confuse equations for the operators with equations for thermodynamic quantities. 3) The operator that ...

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I've heard that some physicists think that the net energy of the universe is zero. Me too. They talk about gravitational energy being negative. But see Einstein talking about gravitataionl field energy here. It's positive. For this to happen, I would assume that the negative gravitational energy of a body ought to cancel out its rest energy. That's what ...

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My understanding of the current theory is that galaxies are moving away from each other at an accelerated rate due to dark energy repulsion--creating an expanding universe. However, within galaxies, dark matter keeps the galaxies themselves together--so much so that the outer rim of the galaxy spins at the same rate of the inner rim--meaning there must be ...

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The ultimate fate of the universe (and here I'm taling about things on cosmic scales not what happens to stars and galaxies etc.) depends on the equation of state of the material within it. In cosmology the equation of state is represented by a dimensionless number that is the ratio of the pressure to the (energy) density. i.e. $$w = \frac{P}{\rho}$$ and ...

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This part of Lawrence Krauss' brilliant "A Universe from Nothing" lecture describes the fate of an expanding universe quite well. Long story short, because the expansion of the universe is accelerating, in a hundred billion years or so the rate will exceed the speed of light (this doesn't require objects to move through space faster than light, that's ...

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I believe that the ever expanding universe is the most popular theory as of now. But what is making the universe accelerate? We believe it has to do with Dark Matter and Dark Energy but no conclusive research has been done yet. So in general, we just don't know what will be the end fate of the universe because there is too much unknown about what is ...

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The size of atoms is determined by the strength of the electromagnetic force, the mass of the electron and some other constants like the value of Planck's constant. If the size of atoms was changing it means that one or more of these constants must be changing. The trouble is that these fundamental constants crop up all over the place in physics, and if ...

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