# Tag Info

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These aspects of astronomy and cosmology are indeed very interesting and very significant, but don't allow the names to get in the way of your understanding. Dark matter is a form of matter made (most likely) of particles which don't interact very much with the matter we are more familiar with (i.e. protons, neutrons, electrons etc.). The evidence for it has ...

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That's simply because there the dark matter density in galaxies is high, but the density in intergalactic space is low. The reason for that, in turn, is because dark energy is a property of spacetime itself - any spacetime, wherever it is. In contrast, dark matter like normal matter follows gravitational forces and clumps into galaxies and other such ...

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Your first sentence is already not defined. If by "standpoint of a light ray" you mean "a hypothetical observer travelling at the speed of light", you are already in undefined territory. The Lorentz transformations, which mathematically tell us how to change between inertial reference frames contain a factor: $$\gamma=\frac{1}{\sqrt{1-v^2/... 3 As far as we know, the Universe does not have a boundary. But, if it does, then we will need to know what are the boundary conditions in order to answer your question of what a particle will do when it reaches it. For example... The Universe could have reflecting boundary conditions, in which case the photon would bounce back from the boundary. The Universe ... 3 So what is the flaw in my reasoning? The flaw is that the FLRW spacetime is not a vacuum solution. Indeed, as you correctly point out the one you wrote down has positive scalar curvature. A vacuum solution has a zero Ricci tensor and therefore a zero scalar curvature. Physically, the FLRW spacetime represents an isotropic and homogenous distribution of ... 2 The redshift that is measured is the sum of the redshift due to the expansion of the universe, a Doppler shift caused by that component of the peculiar velocity that is radial and a second order (negligible) Doppler shift due to the tangential component of the peculiar velocity. It is not possible in general to separate out these effects. The exception is ... 2 The peculiar velocity of a distant galaxy can affect the overall red shift. For example there are local galaxies like Andromeda that are slightly shifted blue because they are advancing. 2 A genuine derivation of the Friedmann equation would go through general relativity. You would start with the spacetime metric, which determines the curvature k, and then evaluate the Einstein field equations to get the result. When gravitational effects are weak, general relativity reduces to Newtonian mechanics. Therefore, in some limits, it should be ... 1 Your model works quite well in Newtonian gravity. You can even derive the Friedmann equations describing the rate of expansion of the universe from your model, and they match the equations from real cosmology, in the appropriate c\to\infty limit. If you adapt your model to general relativity, you get the standard cosmological model. If you start with a ... 1 You're right that in the case of matter moving away at different velocities from a given point, any observer would be moving away from any other observer at a velocity proportional to the distance between them, i.e. Hubble's law would also be true. If this scenario were true, it would mean that our Universe wouldn't be described by general relativity (GR), ... 1 Both experiments report H_0, the Hubble parameter at redshift z=0, now. The different experiments directly measure H(z) at different z's then use the standard model of cosmology to determine H_0. Late time or local measurements, like those using Type Ia supernovae and a cosmic distance ladder, measure H(z) for small z\sim 1. Early time ... 1 Both sets of experiments are measuring the same quantity: H_0, the Hubble parameter today. When people say that Type Ia measure H0 "locally" they mean that the observations are happening at a small redshift. On the other hand, the surface of last scattering was generated at a redshift of about 1100. Nevertheless, both sets of observations are ... 1 Hubble law states that galaxy receding speed is :$$ v = HD $$Acceleration by definition is speed change over time, so :$$ a=\frac{dv}{dt}=\frac{d\left(HD\right)}{dt} $$Applying product rule gives :$$ a = D\frac{dH}{dt} + H\frac{dD}{dt}$$Usually Hubble parameter change over time is expressed as :$$ \frac{dH}{dt} = -H^2(1+q) $$Substituting it into ... 1 I think there are two different questions in your question. Or maybe it is better to say that before asking your question, another question should be considered first. Your question: Can we estimate where the center of the universe is? You also ask about the rate of expansion but I think we have to address the question above first. And maybe your question ... 1 Yes, gravity can slow down the expansion of the universe. That's why if the universe's average density is greater than critical, it is forecast to collapse into a big crunch (see this section of the Wikipedia article on the Friedmann equations). I don't understand your second paragraph very well, but it seems like a misconception about what actually is ... 1 Gravitationaly bound objects, such as the galaxies in the local group (Milky Way and Andromeda for instance) don't destroy the space between them, rather space doesn't expand between them. The expansion of the universe was slowed by the gravity of the objects in the universe until 5 billion years ago when it was dissipated enough that dark energy began to ... 1 I think an example should help illustrate what's going on. First, there is no discontinuity when inflation ends: the rate of expansion goes smoothly from accelerating to decelerating, much like a ball rolling down a hill. To start, consider the Friedmann equation,$$\frac{\ddot{a}}{a} = -\frac{4\pi G}{3}(\rho + 3p),$$where p = w\rho relates the pressure ... 1 how does the dark energy affect the universe (why we usually related them with universal expansion) To answer your second question, you need to look at the acceleration equation.$$\frac{\ddot a}{a}=-\frac{4\pi G}{3c^2}\sum_i(\varepsilon_i + 3w_i\varepsilon_i)\frac{\ddot a}{a}=-\frac{4\pi G}{3c^2}\sum_i\varepsilon_i(1 + 3w_i) Where $\varepsilon_i$ ...

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I guess you are looking for angular diamater distance. For different curvature the equation takes different forms. See here https://en.wikipedia.org/wiki/Angular_diameter_distance

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A couple of things up front: first, you seem to be talking about the present-era expansion of the universe, not inflation. Inflation, if it happened at all, ended about 14 billion years ago, long before the formation of large structures like galaxies and very long ropes. Second, a galaxy with a recession velocity of c in the present era is going to be some ...

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The figure 4.2 sigma means that the discrepancy between the two numbers is 4.2 times the estimated standard deviation of the difference between the two numbers, under the assumptions that the measurements are independent and the uncertainties have an approximately normal distribution. If I have two measurements $a \pm b$ and $c \pm d$, where $b$ and $d$ are ...

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You have two data sets $A+\delta A$ and $B+\delta B$. To see that these two measurements agree or not we can calculate their difference. Let us call this value as $D$ such that $D=A-B$ and $\delta D = \sqrt{\delta A^2 + \delta B^2}$. For instance, in some measurement, we find that $D\pm\delta D \equiv 0.03\pm0.05$ In this case, as you can see D ranges ...

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