New answers tagged

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Because there is no quantum theory of gravitation, we cannot express with confidence the microscopic view of what is happening. However, at the large scale the expansion of space is described by the Hubble parameter, and the Big Bang model, which is informed by the Cosmic Microwave Background (CMB), which once was 3,000 K,but today is about 2.7 K, which ...


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All the particles that annihilate or decay as they become non-relativistic, heat the photon bath. A decaying particle, like the Higgs, also heats the photon bath by decaying to lighter particles, giving those particles alot of energy. In general, if the neutrinos had decoupled at a temperature $T_\text{dc}$, the temperature of the neutrinos would, at any ...


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Matter and dark matter are also evenly distributed throughout the observable universe, at least on the largest scales. What makes dark energy different isn't that it is uniformly distributed, but that it has a constant density. The amount of dark energy per cubic meter of universe is the same regardless of the total volume of the universe. If the universe is ...


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The preferred frame of reference is that of the co-moving reference frame that defines the Hubble flow. In practical terms that can be defined by correcting any velocity for the observer's motion with respect to the cosmic microwave background. Individual peculiar velocities for galaxies (including our own) are measured in hundreds to thousands of km/s. This ...


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Expanding Universe: The idea of the Universe expanding is often described using the analogy of an inflating balloon. It is tempting as a new physics student to imagine the expansion as galaxies whizzing away from each other through some 'medium', however in actuality it is spacetime itself that is expanding. If we glue some pieces of confetti (representing ...


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After answering several similar questions on Physics SE I have realised I can answer this question myself using the concepts of the (comoving) Particle Horizon, the (comoving) Event Horizon and the Comoving Hubble Sphere. If I want to know the maximum distance I can communicate in an any amount of future time, then I should consider the Event Horizon, which ...


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This sort of theory is still an active research topic as can be seen by running this search, which turns up plenty of recent papers related to the topic, including several in credible refereed journals. It is not widely regarded as complete/correct/accepted, but it is grounded in sensible physics... though to me some parts of it are a bit strange.


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By your assumption, we are talking about the FLRW Universe. Such a universe is by definition isotropic and homogeneous so there can't be any preferred direction at any point. If there were a difference between the CMB frame and the co-moving frame, it would indeed produce a preferred direction at some/most points (the direction of motion of one frame ...


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Maybe the Cambridge Cosmology Lecture Notes can help to answer at least the first question. Besides I think you meant annhilation instead of scattering (?), because these equations are for the Dark Matter $\chi$ annihilation process \begin{align} \chi + \bar{\chi} \Leftrightarrow c + d~~~~. \end{align} However, a system is said to be in kinetic equilibrium ...


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The graviton in string theory is an aspect of closed heterotic strings. The reason for closed strings is that there are two independent modes, called right and left oriented modes. This is in contrast to the open string where modes in both directions are the same, being of course reflected back and forth. We may think of these modes as $a, a^\dagger$ and ...


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I though I would discuss the transition from radiation to matter dominated phases and from there to the dark energy phase. A fair amount of this can be discussed with just Newtonian mechanics. General relativity changes this by some subtle means, but as a coarse grained view, to borrow a stat mechanics term, Newtonian mechanics captures a lot of this. We ...


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The idea behind Boltzmann brains are that if you are willing to wait a long enough time, then very unlikely statistical fluctuations will be observed. One such fluctuation would be for a brain to momentarily fluctuate into existence complete with a set of memories corresponding to, say, your total life experiences to date. The brain would then promptly ...


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The title and the text actually ask two different questions. While Kyle Oman and Thriveth answer the title excellently, I'll address the question in the text which asks "Why did the Universe expand in the first place, before dark energy (DE) started to dominate". The answer to this is inflation (we think). The first fraction of a second after the creation ...


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The short version: The amount of matter in the Universe is fixed, so as the Universe expands, matter density will drop because the same amount of matter will be spread out on more space. Dark Energy, on the other hand, is (by definition) constant or almost constant in density. This means that no matter how dilute the Dark Energy is, if it waits long enough, ...


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To answer your specific question: absolutely none. The Millenium run is a "dark matter-only" simulation. In this sort of simulation gas physics is taken to play a negligible role. All the gas (and stars, indeed all "baryonic matter" as it's called in the jargon) is removed and replaced with additional dark matter. The extra dark matter is added just to keep ...


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Let's start partway through the expansion of the Universe in the matter dominated epoch. At this time the energy density is dominated by matter, but the dark energy and radiation components are still present, just relatively small. The Universe is expanding, but the expansion is gradually slowing down. As the Universe expands, the density of matter scales ...


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These are not easy questions and there is no definite answer for them. There are plenty of cosmological models which we believe to be at least partialy true, and they do adress some of these questions. However, it is difficult to be entirely confident in their validity. I think most of cosmologists believe that there is infinite amount of matter in the ...


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The question isn't entirely clear, but I suspect that you're being asked to prove that the given parametric equation for $R(\theta)$ and $t(\theta)$ satisfies the Friedman equation. If so, you shouldn't try to get rid of $\theta$ entirely. Instead, show that the left-hand side (as a function of $\theta$) is equal to the right-hand side (as a function of ...


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Your question is not specific to inflation, and really applies to any case where a bosonic quantum field behaves semiclassically due to macroscopically large occupation numbers. One very simple example of this is the Stark effect in quantum mechanics, where a Hyrodgen atom is placed in a uniform electric field. The atom is treated as a quantum mechanical ...


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The most distant object that light we emit today can reach in the distant future is at the event horizon $$eH(t) = a(t)\cdot \int_{t}^{t_{max}} \frac{c\cdot \text{d}t'}{a(t')}$$ which is now approximately 17 billion lightyears away, see the future light cone in comoving coordinates which converges to this distance: If the light was emitted at the big ...


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As already explained in other answers and comments in General Relativity (GR) energy is not conserved. Some people and physicists say it is, it simply gets lost by the matter-energy and gained by the gravitational field, or viceversa; this is more pleasing to our sense of conservation of something, but it has problems in that the gravitational energy, is not ...


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The metric for the de Sitter spacetime, which approximates the observable universe in stationary coordinates is $$ ds^2~=~-\left(1~-~\frac{r^2\Lambda}{3}\right)dt^2~+~\left(1~-~\frac{r^2\Lambda}{3}\right)^{-1}dr^2~+~r^2d\Omega^2 $$ The important term is $$ \left(1~-~\frac{r^2\Lambda}{3}\right), $$ that looks a bit like the Schwarzschild factor. This ...


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Well first of all, Considering dark energy and other factors, what is the most distant object light could reach? If you are talking about an OBJECT like stars, galaxies etc... the farthest object we can "see" is located 13.39 bilions light-years (Galaxy GNz-11, you can search that) The 11 on the name indicates its redshift z=11. ...


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The answer is that we don't know. Why? Because the theory of gravity which we have and use, GR, has a singularity. Things which should be finite in a physical theory, like the density, become infinite. And theories with a singularity are simply wrong, they need a modification, and this modification is necessary not only at the singularity itself, but already ...


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In the Standard Model, the baryon and lepton number are accidental global symmetries. However, they are conserved only at the classical level: quantum corrections do not respect them, i.e., they are anomalous. The interesting thing is that they are violated by exactly the same amount. In terms of currents we write: $$\partial_\mu J_B^\mu=\partial_\mu ...


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Although the radiation-dominated (RD) era is long in comparison to the matter-dominated (MD) and $\Lambda$-dominated ($\Lambda$D) eras, it is nice to have an answer which can be adapted easily for any cosmological era. If we assume that the Universe is permeated by a perfect fluid we may use the equation of state \begin{equation} w = \frac{P}{\rho}, ...


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I will use most of @Ted answer to describe 'hot' but I will ask a more basic question: I think the best way to think about it is that the sentence "the photons have cooled" is simply describing a fact, not explaining that fact. At early times, the photons at any given location had a thermal (blackbody) distribution corresponding to a high ...


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A fine tuning problem is only a problem if we require that the considered model is a good or complete model of how we think the universe behaves. In this case, it appears that we require the density of the universe to be as close as 1 part in $10^{64}$ to the exact density that will make it a flat universe that will expand forever, asymptotically coming to ...


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The argument is that (mixing happens) => (inflation happens) => (mixed regions are out of causal contact, but have no way to change their local temperature) In this scenario, it doesn't matter when the photons are emitted. Their apparent homogenity is an effect of the left most step.


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Poincaré theorem holds for Hamiltonian systems of finite phase space. We do not know whether Universe is finite or infinite and thus whether it is better to describe it with mechanical model of finite or infinite phase space.


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The Gödel universe is homogeneous and every observer anywhere in the universe observes the universe to be rotating around them. So a Gödel universe has no centre.


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It changes. In fact, after 3000 billion years, the constants will have changed so much that all the current structures in the universe will be destroyed, including quarks and electrons.


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The process by which particles are created after inflation is known as "reheating". One needs a coupling between the inflaton and another particle's field for this to happen. Generally it occurs at the end of inflation when the inflaton is oscillating in the well of its potential and the expansion rate falls below the interaction rate between the inflaton ...


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Welcome here. From your profile I see that you are at the beginning stages of learning physics. This is an arduous process that needs a lot of elbow grease in solving problems and/or doing experiments in order to get a basic intuition for the subject. Here is a simplified answer to your questions: Why do most theories about what Dark energy and Dark ...


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First of all, the Universe isn't expanding according to "current theories". It is an observational fact. Second, there is no center of the Universe. Space was created, and started expanding. This expansion pulls everything away from each other. Galaxies lie approximately still in space, but space is expanding. This means that no matter where you are located ...


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First of all, there is NO centre in the universe. I know it's not a good analogy, but think of the universe as the surface of a balloon. Forget the interior, we're only looking at 2 dimensions, whereas the real universe has 3 of them. Put some ink dots on the balloon, which represent galaxies (note: NOT planets). Now inflate that balloon. You'll see that ...


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Ok, lets look at how we determine $\mu$ in a cosmological setting. In order to determine $\mu_i$, we can use the fact that, in equilibrium, $\mu$ is conserved in all reactions. This means that if we have a scattering process $i + j \rightarrow a+b$, then we know that $\mu_i + \mu_j = \mu_a + \mu_b$. Fermions in equilibrium, like electrons and neutrinos ...


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Therefore you need to calculate the future light cone $$LC_{proper}=\int_{a(t_0)}^{a(t_1)} \frac{c\cdot a(t_1)}{\alpha^2\cdot H(\alpha )} \, \text{d}\alpha$$ In comoving coordinates you divide that by the scale factor of the time at absorption $$LC_{comoving}=\frac{LC_{proper}}{a(t_1)}$$ with H as the Hubble parameter $$H(a)=H_0\cdot ...


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The inflationary theory you mention is probably eternal inflation. In this theory there is just one universe but different parts of it are causally disconnected i.e. the different parts cannot affect each other in any way. Whether these constitute a multiverse comes down to terminology. In principle there is a continuous spacelike straight line that links ...


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It's a confusion of terms. The universe is a closed thermodynamic system whether or not it is 'open' or 'closed' in a cosmological sense. In cosmology, open and closed universes refer to the curvature of the universe, whether positive (closed, finite universe), zero (flat, open, infinite universe) or negative (curved, open, infinite universe). In ...


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If the universe is open, there's obviously more universe that you haven't included in your system. The universe, by definition, contains all energy and matter. An open system, by definition, has an outside system to exchange energy and matter with. If that outside system isn't part of the universe, then where is it?


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The scale-factor of the universe changes significantly over that period of time, so you can't calculate distance as simply $d = v \cdot t$. You have to actually integrate over the expanding spacetime metric, i.e. $$s = \int_{z_1}^{z_2} \frac{c_s}{H(z)} dz$$ Where $c_s$ is the speed of sound (and I think that should be $c/\sqrt{3}$ instead of $c/2$, ...


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We want the Newtonian limit of the Einstein Field equations for nonzero vacuum energy(=cosmological constant). As $\rho_\mathrm{vac}=\Lambda/4\pi G$ is a mass(=energy) density, Poisson equation is $$ \Delta\Phi=4\pi G\rho(\boldsymbol r)-\Lambda \tag{1} $$ If we assume spherical symmetry, and point-like source $\rho\sim\delta(\boldsymbol r)$, the ...


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You get an extra term that increases with r: $$a = -\frac{G\cdot M}{r^2} + j\cdot r$$ with j as the repulsive component.


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Here's another, simpler answer. The BB analyses (like the one above) are typically based on some very modest definition of life, like an unpressurized brain floating in vacuum. If you include all the other equipment necessary for a brain to actually function then the odds are much different. Furthermore, the universe actually doesn't contain much ...


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In the geometrical optics approximation light ray is represented by a null geodesic. Therefore you only need to find a null geodesic connecting points $(t_0,0,0,0)$ and $(t_1,x,0,0)$ for some $t_1$ (and this condition will determine $t_1$ uniquely). This is probably quite easy to do directly in this case, but in general for investigation of null curves in ...


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EDIT: My first answer seemed to imply that radiation is at rest in the Cosmic Rest Frame. Radiation is not in rest in any frame. See below. The sentence shouldn't be read as "[velocity of energy] forms", but "velocity of [energy forms]"$^\dagger$. The sentence refers to "energy forms", i.e. the different forms in which energy can manifest itself. These ...



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