# Tag Info

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Have a read through Did the Big Bang happen at a point? and the answers to it. The singularity at the Big Bang is the zero time limit of the equation (the FLRW metric) that describes the expansion of the universe. Most physicists believe that this is a mathematical artefact and does not describe what actually happened. It seems likely that some quantum ...

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No, there is no contribution to the pressure from the gravitational attraction between the particles. To see this you need to appreciate that the pressure is an ensemble property, and look at the stress-energy tensor for a single point particle. This is: $$T^{\alpha\beta}({\bf x},t) = \gamma m v^\alpha v^\beta \delta\left( x - x_p(t) \right)$$ where $v$ ...

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One. The inflationary period is thought to have lasted from around $t = 10^{-36}$ seconds to $t = 10^{-33}$ seconds after the Big Bang. So while you're technically correct to say it lasted less than a second that's a bit of an understatement. Two. See my answer to What was the density of the universe when it was only the size of our solar system? for the ...

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All statements like "when the universe was the size of a grapefruit" refer to the currently observable universe. As the universe has a finite age and light travels at a finite speed (and there is nothing infinite going on with expansion), the observable universe is a finite patch. I discussed some of the different notions of horizons in answering another ...

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The steady state theory fails to model a few observed features of the universe: the accelerated expansion of the universe radio galaxies and quasars that are only observed at high redshifts & not everywhere the existence of the microwave background light

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Here is a good basic summary of the history of the steady-state theory and the observations that caused it to fall out of favor, mainly the second two mentioned by Kyle Kanos. One of these was the observation of intense radio sources that didn't seem evenly distributed throughout the universe, but were only seen at large distances (higher redshifts): The ...

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What caused the transition,[...] As the universe expands, the number of particles per unit volume goes down. But in addition to this, photons suffer a cosmological red-shift. So the density of mass-energy due to nonrelativistic particles goes down, and the density due to photons goes down, the latter goes down faster. By extrapolation, we predict that ...

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If we take the simple approach of determining the state of the "jerk" today by assuming an exponential expansion (e.g., $a(t)\sim\exp(H_0 t)$), then $$\dot a=H_0a\tag{1}$$ The derivative of this is then, $$\frac{d^2a}{dt^2}=H_0\dot{a}=H^2_0a$$ And now for the "jerk," $$\frac{d^3a}{dt^3}=H^2_0\dot{a}=H^3_0a\tag{2}$$ The Hubble constant is already pretty ...

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The total energy of the universe is a vexed issue since different commentators have different views about what the concept means. See the question Total energy of the Universe for a sampling of the various viewpoints. If you Google for zero energy universe you'll find several papers purporting to show that the total energy is zero. However since their ...

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So I've done some further research into this question and the result I found is quite surprising. There truly is no set definition. Some cosmologists will tell you (as John Rennie mentioned) to avoid using the term "Big Bang" unless you absolutely have to. However, that is a luxury not afforded to all cosmologists. The more surprising thing is that among ...

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I had a quick look at the paper - its mostly nonsense. The intrinsic light from a quasar is completely dominated by its emission line spectrum and a mostly featureless continuum. The emission lines give the true redshift of the quasar. Absorption lines in quasar spectra are predominantly due to foreground gas clouds at lower redshifts than the more distant ...

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The basic reason for cosmological expansion is simply inertia. Because the universe was in an expanding state soon after the big bang, it kept expanding. This is roughly analogous to Newton's first law of motion. In addition to this, dark energy is currently causing a significant acceleration of the expansion. (Its effect was not dominant in the past, and ...

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We don't think the FLRW metric is valid throughout the entire history of the universe. If we take a metric of the form: $$ds^2 = -dt^2 + a^2(t) d\Sigma^2$$ then we expect this to be valid throughout the history of the universe as long as the universe is isotropic and homogenous. However we need to find the equation for the function $a(t)$, and this is ...

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The F(L)RW metric comes with very few assumptions, though these are fairly strong: Spacetime is homogeneous. Spacetime is isotropic. Or, in other words, the cosmological principle is assumed. Philosophically this is very desirable, as the notion that there are preferred locations or directions in the Universe is, from a modern point of view, somewhat ...

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At what speed does our universe expand? This question doesn't make sense in the form in which it was posed. To see why, let's start by thinking about how we know the universe is expanding. The expansion of the universe was originally discovered by Lemaître and Hubble, who found that the redshifts of galaxies were proportional to their distances from ...

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Only in the case of a static spacetime is the metric derivable from a scalar potential. Cosmological spacetimes aren't static, so they can't be derived from a potential.

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Your questions are in no way challenging. The answer to both questions is the same: it could be, we do not know. Actually, you could also ask the opposite: how do we know that physical space is equivalent to the continuum (the real line) instead of being a larger infinite (by this a mean an ordered field of larger cardinality, such as the surreal line)

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Ever heard of the cosmic microwave background? The CMB is a relic from when the universe became "opaque" - when, as Wikipedia says, protons and electrons combined to form neutral atoms. These atoms could no longer absorb the thermal radiation, and so the universe became transparent instead of being an opaque fog. So photons decoupled and the CMB was ...

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The following passage has been extracted from the book Parallel worlds: Finally, in Nature magazine in 1965, Hoyle officially conceded defeat, citing the microwave background and helium abundance as reasons for abandoning his steady state theory.

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To my mind, the Big Bang doesn’t refer to a distinct event but to a cosmogonic theory as a whole, that “predicts” ( should we say “retrodicts”?) many different events of the deep past. For example, there is such established term as “Big Bang nucleosynthesis” that describes an epoch several seconds past the Beginning of Time. The Beginning of Time in the ...

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There is no definite SIZE to the universe as such. There is however a size to the OBSERVABLE universe. These are very different. And indeed the observable universe is defined by Einstein information caveat where information cannot propagate faster than light. Now, it is in this sense, that Newtonian mechanics fails us, as newtonian gravity (and grav. ...

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Ulitimately the Universe's expansion is due to the initial conditions, unfortunately explaining why these initial conditions exist is beyond the scope of classical big bang theory as they exist as parameters than can be adjusted. However the expansion of the Universe is not independent of the matter it contains and the Friedmann equations link the rate of ...

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Black body radiation is a statistical description i.e. it assumes there are enough photons that they are distributed according to Boltzmanns law. At energies high enough for a single photon to equal the total energy of the system this assumption breaks down and the black body description will no longer apply. But by the point the energy has got this high ...

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You presume causality, namely that something has to occur to instigate something else (cause and effect). In fact, you presume that there is a well-defined "time". However, our current best theories have problems defining time close to the start of the universe. If there is no clear way to define time then you cannot say that something has to precede ...

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