# Tag Info

## Hot answers tagged universe

4

In curved spacetime, you can no longer compare velocities at different points in the straight-forward manner we use in flat spacetime. Thus the claim that recession velocities should not be considerer 'real' (as in relative) velocities, but rather rates of expansion of space. If you want to get at the former, you need to parallel transport the source's ...

4

Why can't we set up a sphere with photodetectors with synchronized clocks To anyone moving relative to the sphere, the clocks aren't synchronized (relativity of simultaneity); the synchronization of spatially separated clocks is reference frame dependent and this is, in fact, the root of time dilation and length contraction. Moreover, the process of ...

3

The dividing line meets at $(\Omega_m,\Omega_\Lambda)=(0,1)$. From the Friedmann equations, it follows that the scale factor $a(t)$ satisfies the relation $$\frac{\dot{a}^2}{H_0^2} = \Omega_m a^{-1} + (1 - \Omega_m - \Omega_\Lambda) + \Omega_\Lambda a^2.$$ The universe has no big bang singularity if the above expression is negative (or zero) for some ...

3

It is only in the absence of dark energy that the correspondence between geometrical curvature and the ultimate fate of the universe is as straightforward as you describe. Measurements (primarily of the cosmic microwave background) indicate that our universe is flat or very nearly so, which should be interpreted geometrically (i.e. in terms of the sum of ...

2

All the photodetectors will receive the flash at the same time. Start by ignoring the expansion of spacetime: you've already established that the speed of light is the same for all observers, so the light at the centre sees the light moving away at $c$, and all the detectors see the light approaching at $c$. Since all the detectors are the same distance ...

2

The rate of formation is much higher in the presence of dust. There needs to be a mechanism for the energy of formation of the hydrogen molecule to be dissipated. Dissipating energy via a photon involves a forbidden transition. Instead, the energy can be transferred to the vibrational lattice of a dust particle. See The Interstellar Abundance of the ...

2

Think of a very thin wire. It is a 3-dimensional object, but for many purposes you can describe it just as a 1-dimensional line or curve. The two remaining dimensions are curled up in a tiny cross section. In a similar way, the speculations (not the slightest experimental hint exists that it should be so) about our world possibly being higher dimensional ...

2

You are thinking that the big bang happened in a particular point in space and then expanded outwards from that point. This is not true. The big bang happened at all points in space. This is because space itself expanded in the actual bang. Therefore each point in space has its own "horizon" of 13.7 billion light years across. This edge is due to light ...

2

I think you are missing that the source is actually referring to he observable universe explicitly - just somewhat indirect: It is not obvious as it is talking about "visible/observable universe", but about "total mass of the visible matter" and "mass density of visible matter". As far as I can see, any references to mass etc in the universe are covered by ...

2

I read this question but I didn't understand the physics equations used in the answer. Let me offer a simplified explanation. The origional Big Bang cosmology asserts that the universe is expanding, which if true means that the proper distance $D$ between any 2 points is increasing. Since you can think of this expansion as "space itself expanding," then ...

2

The size of the universe is given by a scale factor, normally written as $a(t)$, that is a function of time and is calculated by solving Einstein's equation for an isotropic homogeneous universe. Once we've calculated $a(t)$ we can differentiate it wrt time to get $\dot{a}(t)$ and use this to calculate the recession velocities. The scale factor is a ...

1

The Milky Way is receding from the members of the Hydra-Centaurus Supercluster. The Hydra cluster has a red shift of 0.0548. The Centaurus cluster has a red shift of 0.0114. The Norma cluster has a red shift of 0.0157. The local group is and will continue moving away from the Hydra-Centaurus Supercluster.

1

Space itself was once concentrated in an infinitesimally small point. During the Bang of the Big Bang all distances between points got bigger. If you try to measure the expansion of the universe from any point you will draw the conclusion that the expansion started from that point. It seems that the expansion happened everywhere, and nowhere at the same ...

1

No. The standard metric for cosmology is given by: $$ds^{2} = - dt^{2} + a(t)^{2}\left(d^{3}{\vec x}^{2}\right)$$ where the term inside the parenthees represents the 3-metric of a homogenous three space. As you can see, there is no difficulty with evaluating the age of the universe: $$T = \int\sqrt{-g}\,\,x^{a}y^{a}z^{a}\epsilon_{abcd} = \int dt$$ ...

1

The "scientific view of creation" is the Big Bang. It's as widely accepted as gravity. Nobody doubts it exists, we're mostly discussing how it works. As for the conservation of energy, that basically states that at two points in time the total amount of energy is equal. You need those two points, though. And there is no proof that there is a point in time ...

Only top voted, non community-wiki answers of a minimum length are eligible