# Tag Info

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Cosmology usually adopts something called the "Cosmological Principle", which is that, on large scales. the universe is homogeneous and isotropic. Therefore the universe looks the same wherever you are and looks the same in all directions. Thus light emitted from our part of the universe travels outwards and is received by distant parts of the universe ...

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If this is so, then isn't light constantly getting past all matter in the universe and so being lost. It may be, but we have no way to know. We do not see far enough to see what happens on the edge of the material universe. Even if there is such edge and only vacuum beyond, energy of the material universe does not need to decrease, because there may ...

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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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$H$ tells us how fast the universe is expanding, relative to how much it has already expanded. It has units of inverse time. For example, if $H=0.1\ \mathrm{s}^{-1}$, then the universe is expanding by 10% every second. Suppose that the density of mass-energy in the universe was so small that deceleration was negligible, and suppose that at $t=1$ s, we have ...

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Gravitational redshift is extremely small for stars, it has no significant impact on cosmic redshift measurements of galaxies. The gravitational redshift of light emitted by a star is of the form $$z = \frac{1}{\sqrt{1 - \frac{GM}{Rc^2}}} - 1 \approx \frac{GM}{2Rc^2},$$ where $R$ is (approximately) the radius of the star. For the Sun, this is of the order ...

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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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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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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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As I stated in my comment, our observable universe is much larger than the Hubble radius: we can observe galaxies that are receding from us faster than the speed of light. I refer to this post of mine and links therein for more info: http://physics.stackexchange.com/a/63780/24142 Also, in the standard cosmological model, where the density of dark energy is ...

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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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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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You are apparently comparing with the expansion of some object, which is being heated. Well, such an object consists of particles, atoms in a lattice maybe, and each of those vibrate. More thermal energy makes them vibrate more violent, which make them fill more space. If all atoms require more space, the object expands, since they all "push" at each ...

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