# Tag Info

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That depends on what you mean by "made up of". Is static electric field "made up of" photons? Is the stream of water "made up of" waves? To me these are semantic games that have no connection to reality. But if you are willing to answer "yes" to the questions above, then you can safely say that those domain walls are "made up of" Higgs bosons (and also the ...

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The basic problem with this is the so-called "cosmological principle". We expect the Universe to be 'homogeneous' (statistically the same no matter where you look, provided you zoom out far enough) and 'isotropic' (the same no matter how you rotate it). Failure of either condition would essentially mean that the Universe has a centre, which would be weird. ...

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If SMBHs were present from the very beginning, I think we would have evidence of that. There would be nothing to stop them accreting and forming quasar-like objects and so the co-moving density of quasars would be high at very large redshifts. But there is clear evolution in the quasar density, peaking at redshifts 2-3. The number density declines by orders ...

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My answer to How does the Hubble parameter change with the age of the universe? (which is itself adapted from Equation for Hubble Value as a function of time) explains how to calculate the scale factor. In fact we calculate the time as a function of the scale factor rather than the other way around. The equation we use is: t(a) = \frac{1}{H_0}\int_0^a ... 0 The cosmological event horizon is the furthest distance light can travel and is also the upper bound to how far a massive particle can travel. A free falling massive particle will asymptotically approach a point which is receding from the point which it started with the same recessional velocity as its original peculiar velocity. I believe the correct ... 0 Just a partial and naive "answer", using the uncertainty principle. An observer makes an energy mesurement in some empty volume V_3 during a time intervall \Delta t. According to Heisenberg uncertainty principle, he wil get an uncertainty \Delta E on the energy mesurement : \begin{align} \Delta t \; \Delta E &\approx \Delta t \; \rho_{\text{vac}} ... -1 To make sense of this question you need to decide what an "object" means (is a rock an object or a conglomeration of a vast number of much smaller objects?). Once you've settled that, you need to decide whether you're averaging velocities or speeds. If there are three objects, and two of them are moving away from me at the same speed v in opposite ... 0 No, there cannot be a most stationary object in the universe. One of the basic tenets of relativity is that every inertial reference frame is equivalent. This is what Einstein said on the matter, If a co-ordinate system K be so chosen that when referred to it the physical laws hold in their simplest forms, these laws would be also valid when referred ... 1 p=\frac{1}{3}\rho is the well-known equation of state of a photon gas. It may be derived by looking at the ultra-relativistic limit of the energy momentum tensor for a bunch of particles.^1 p=-\rho follows from the fact that the energy momentum tensor of \Lambda-style dark energy is proportional to the metric. Thus, at a point and in the proper ... 0 No. A low entropy "initial state" could be the result of a so-called anthropic fluctuation in a (past) eternal universe. Fluctuations about equilibrium could, fortuitously, create the initial conditions for life as we know it. This was proposed by Boltzmann and his assistant Schutz in the late 19th century, though ultimately deemed unsatisfactory by a ... 3 At the classical framework , i.e. no General relativity and astrophysical observations of the 18th century , this is a valid question. When talking of a "Universe" one must have a model , and the model depends on the state of physics knowledge at the time of the model. The second law states that entropy always increases or stays the same. One can make a ... 0 Comoving observers move along with the Hubble flow, and perceive the universe as having no Hubble expansion, due to an increasing scale factor. So, when you invoke a comoving observer, it's not surprising that the observer sees no redshifting. -1 Total entropy of the universe is equal to the total area of the space boundary, according to holographic principle. Our universe is expanding, asymptotically approaching de Sitter space. In de Sutter space the radius of the cosmic horizon is constant and equal to the Hubble radius - the distance at which cosmological red shift becomes infinite. For ... 0 You are correct that the gravity of everythig in the Unuverse should have contracted the universe from everything we know, but the fact is we don't have all the answers. For example, what caused the big bang? What cause the sudden inflation? what is dark energy? String theory has some potential answers for the initial inflation but string theory has run ... 1 Your question assumes that the universe started out as a point at the Big Bang and then expanded outwards, however this is not the case. Have a look at my answer to Did the Big Bang happen at a point? for more on this. However your main point remains, that is shouldn't gravity be slowing the expansion, and indeed it did until a few billion years ago. The ... 0 The particle horizon is just the distance moved by a light ray in time t. To calculate this we start with the FLRW metric. We'll consider only radial motion so d\theta = d\phi = 0 and the metric simplifies to: ds^ = -c^2dt^2 + a^2(t)dr^2 \tag{1} $$For a light ray ds = 0 and substituting this in equation (1) and rearranging we get:$$ ...

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The infalling observer can 'see' whatever events are in its past light cone. The past lightcone of the infalling observer at the point of intersection with the horizon does not enclose the entire exterior region. In fact, no point on the infalling trajectory does, even at the singularity. Therefore the infalling observer unambiguously does not see the "end ...

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Yes there is. The solution to the Friedmann equation in a flat universe with a cosmological constant is $$H^2 = \frac{8\pi G}{3}\rho + \frac{\Lambda}{3},$$ Thus, as the universe expands and the relative importance of gravitating matter, characterised by density $\rho$, decreases, then $\Lambda \simeq 3H^2$. We are already (just) in a dark energy dominated ...

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Determining what is happening "right now" on a planet 63 light years away is exactly the same as determining what will be happening 63 years in the future on Earth. Both problems are, technically speaking, impossible. We can't know exactly what the future holds 63 years from now, as things may change in surprising ways, just as we can't know what is ...

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The values of individual entries in the metric tensor depend on the coordinate system you choose. In the case of the FLRW metric there is a natural choice of coordinates called the comoving coordinates. In particular the comoving time has a very simple interpretation because it is equal to the proper time of a stationary observer, which obviously means ...

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I think it has to do with their relative intensities and the high density of food compared to the low density of the universe, This is correct. The microwave ovens are not working with a black body radiation curve with an average of 3K , nor is the heating effect a thermal balance between two black bodies: food and microwave. Bodies in space away from ...

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The reason that there is a CMB is because of the big bang. The photons from the very beginning of our universe has spread (almost)uniformly throughout the universe to give rise to a general noise which we call background. Now to answer your question, as the universe expanded after big bang the photons got redshifted and their energy decreased. Now the avg ...

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First of all, the universe is not infinite. There is enough reason to disprove an infinite universe model. In such a case, the universe can have a beginning. Reasons to why the universe simply cannot be infinite are as old as the time of Isaac Newton. The problem is gravity. In an infinite universe, the total gravity of the mass it contains would be infinite ...

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you cannot look at the expansion of the universe -- or say the big bang -- as being comparable to an explosion. Every point in space drifts away from every other point in space. This is different to an explosion, where you have a real center. The latter aspect is missing -- or say different -- when you look at the expansion of the universe. Here you don't ...

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You are correct, the surface of last scattering is different for different observers. Its distance is approximately 14,000 Mpc. For a given feature, The change in angle from one position to another is given by parallax, and for small angles is approximately $\Delta \theta=d/R$, where R is the distance to the surface and d the distance between vantage ...

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Dark matter has been postulated to explain discrepancies in observations Astrophysicists hypothesized the existence of dark matter to account for discrepancies between the mass of large astronomical objects determined from their gravitational effects, and their mass as calculated from the observable matter (stars, gas, and dust) that they can be seen to ...

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While I can't speak to the specific example listed, or the particular meaning of the $S$ term, there are examples of spacetimes that agree up until the singularity. First consider a spacetime that is topologically $\mathbb R^4$ with time being the radial coordinate and then for each time you get a three sphere where you then adjust the scale factor in the ...

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A space train leaves Mars at 14:00pm and arrives on Earth at 19:45. The train moves at 0.001C and has 40Km of length. How long will it take for the whole train to arrive on Earth? - Disregard re-entry and friction. Nobody on Earth will say the train is leaving mars now. Same thing with the light, just it moves faster and is smaller than the train above. ...

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As far as I know there is no universally agreed definition of material system and I suspect different authors will use the term in different ways. In my experience it means anything that has a non-zero stress-energy tensor, so it includes any distribution of matter and energy. This would include any arrangement of photons. However it would exclude things ...

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A physicist, me for example, identifies events by choosing a set of coordinates. For example I have a clock that I use to record time and a ruler that I can choose to measure distance. This allows me to set up some coordinates $(t, x, y, z)$ so I can assign every event to some point in my coordinate system. If I received a laser pulse from Mars at 16:05 ...

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For what i know the "infinite size level 1 multiverse" naturally emerge from the eternal inflation picture . The fact that the universe can appear infinite in size from an inside observer while appearing finite in size from an outside observer has to do with the concept of time. More in detail ;the concept of time when measured in different locations with ...

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The Big Bang is a mathematical model of how the observable universe evolved , based on fitting astrophysical observations. Like all models it has its region of validity. When I read cosmology fifty years ago, the model included a singularity at the origin, because that is what the mathematical functions of the General Relativity solutions showed. The model ...

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Although not a complete answer, one place to start is with the coldest naturally occurring place in the universe, which is the Boomerang Nebula, a planetary nebula that is around 1 K. As best as I can tell, this cooled below the CMB temperature simply by adiabatic expansion, and is insulated in its interior from CMB heating. Is this a feasible way to get to ...

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There is another big problem with ultra-small luminosities: due to the small initial light + 1/r² decrease, it might be that only a few photons per hour sent by your target planet reach the diameter of Earth (better be in your telescope ! ). At very small luminosity you have to remind that light is not continuous and made of photons. And way before the ...

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The spatial resolution of a telescope is going to be limited in what it can resolve by something called the diffraction limit. Basically, light can only be focused so much by a lens given its initial starting size and the focal length of the lens. Its useful to think about this in terms of angular resolution for the case of telescopes, and the minimum ...

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Questions like this are complicated because you have to be clear what you mean by time. The simplest definition is that time is what is shown on a clock, so if I was holding some hypothetical clock that had been reset to zero at the Big Bang my clock would currently be showing 13.799 billion years i.e. the age of the universe. The question then becomes what ...

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This is true for galaxies beyond the cosmologic horizon. BTW they are not moving at that speed: their apparent speed seen from our place is such. Quite like the fact that the far galaxies we see close to the horizon seems both very young (which they aren't "in real life"), very red-shifted (while their emitted colors are indeed normal) and living very ...

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The precise shape and medium details of our galaxy are very unknown: we are at the worst possible place to figure them, even if the first survey try to figure roughly our spirals and (vaguely) sub-spirals. So we won't recognise our galaxy. But we might recognise it's neighborhood (close as you say, or far), and deduced it's us at the middle. Still if you ...

3

The universe, as far as we can see it, is fairly uniform, with no edge and no favored direction (Axis of Evil being a possible counterexample, but not here relevant). So, each galaxy has galaxies on all sides of it, all radiating energy towards it. The reverse is also true: each galaxy radiates energy in all directions. The net result is that the radiation ...

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have a read through Did the Big Bang happen at a point? as this provides important background. If, as you say, you are considering only a simply connected universe, so it isn't finite due to its topology, then the assumption we make when solving Einstein's equations is that the universe is the same everywhere - the technical terms are isotropic and ...

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The FLRW metric starts with "the assumption of homogeneity and isotropy of space". But Einstein described a gravitational field as space that is "neither homogeneous nor isotropic" : Hence setting expansion aside, for the universe as a whole there's no overall gravitational field, and light goes straight. Because of this we say the universe is flat, as ...

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Definition 1. A spacetime is said to be spatially homogeneous if there is a one-parameter family of spacelike hypersurfaces $\Sigma_t$ foliating the spacetime such that for each $t$ and for any points $p,q\in\Sigma_t$ there is an isometry of the spacetime metric $g$ which takes $p$ to $q$. Definition 2. A spacetime is said to be isotropic if at each point ...

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The universe, on a larger scale, is of course expanding, but it doesn't really mean that none of the two galaxies should be allowed to run into each other. Because expansion of the universe is not the only factor that will decide the relative separation between the galaxies. The other major factor which affect the relative separation between the galaxies is ...

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All galaxies are indeed accelerating away from each other, but that doesn't mean that galaxies in their direction won't run into other galaxies.

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The elements that make up the bulk of the Earth were part of the presolar nebula. A similar (though not identical) mixture of elements is found in meteoritic material, which is thought to more accurately represent the mean abundances of that nebula (minus the volatiles) and indeed also agrees with the abundance patterns in the Sun. There are grains of ...

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However, if the universe is infinite and unbounded and uniformly populated, there is no empty volume for the galaxies to move into. Therefore logic dictates that the space between galaxies must be expanding. Your premise is faulty. Just because something is infinite doesn't mean it can't expand. For example, although it's intuitively obvious that the ...

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A Galaxy cluster could have $10^{14}$ solar masses within a radius of 5 Mpc. In this case $GM/Rc^2 \sim 10^{-6}$, equivalent to a velocity shift of less than 1 km/s. Our own Milky Way has a mass of around $10^{12}$ solar masses within 100 kpc. This gives a gravitational redshift of about 100 m/s. These are completely negligible compared to cosmological ...

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The gravitational red shift is only significant for black holes – where the coefficient may grow arbitrarily large in the vicinity of the horizon – and the neutron stars – where the frequency drops to something comparable to 50%. For all other celestial objects, the red shift is much smaller than one. And only planets and white dwarfs are objects for which ...

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