# Tag Info

## Hot answers tagged cosmology

14

Light from recombination is not "constantly shining" and that's why you see it. At a given time in the universe's history (actually a slightly extended period but I'll keep things simple), and only at that time, photons decoupled from the ambient plasma and started travelling freely from all points in the universe. The photon background you see at any time ...

11

What you're asking about is the existence of surfaces of simultaneity. In SR, surfaces of simultaneity can be defined by measurement procedures such as Einstein synchronization, and they turn out to depend on one's frame of reference. In GR it gets a lot tougher to do this. We don't even have global frames of reference. It turns out that what you need in ...

10

It sounds like you're interested in when a spacetime admits a Cauchy surface. The answer is that every spacetime that is globally hyperbolic has this property. This was proved by Geroch in 1970 (article here, see Section 5). This includes most of the textbook relativistic spacetimes --- Schwarzschild, Kerr, FLRW, and many others. But there are some ...

9

Earth can lose heat to space through radiation. The earth behaves roughly as a blackbody and so radiates electromagnetic radiation into space at a rate of roughly 120 PW.

6

Let me present a slightly different perspective to Luboš, though I'm saying basically the same thing. From our current location we can define an area of space called the future light cone. This is the region of spacetime that is connected to us by motion at less than or equal to the speed of light. If we draw a spacetime diagram then the lightcone looks ...

6

At a basic level: The universe, in the beginning was very hot. So hot in fact that there were no atoms, only electrons and protons and neutrons and photons flying around. The photons were scatting off of the electrons and protons, as they interacted strongly because the electrons and protons are charged. The universe was much like the plasma you find in ...

5

As for the straight line, yes. All objects will continue moving along geodesics (a straight line in curved-space but sometimes a curved line in straight-space) if there are no external forces acting on them. Unless, by different velocity you mean the direction is not entirely radial to us. In that case, the expansion will cause the object's path to appear to ...

5

The acceleration of the expansion is currently observed to be happening. This observation is one of the pieces of data we use to infer the amount of dark matter. It tells us that there can't be more than a certain amount of dark matter, because that would be incompatible with the observed acceleration.

4

The short answer is yes, the presence of dark matter would act to counter the expansion of the universe. And in fact it does--but not enough to stop the expansion. Dark matter has gravity just like normal matter. In fact, that's pretty much the only reason we know dark mater exists at all: we can observe dark matter's gravitation effects in the rotation ...

4

I will assume you are talking about the center of mass. If there's no external forces, the center of mass would conserve it's momentum. So, it would stay in constant speed, whatever what that speed is, with respect to whatever inertial frame of reference. This happens because Newton's third law. In the summation of all forces, the internal forces will ...

4

I am aware that my answer can sound surprising, too simple to be true, but please take a deep breath before downvoting.The answer has little to do with relativity. In SR it is the moving object that gets shorter , but space is stable. In such a universe, even if a body is receding at 2,3,30 c, its light will reach us sometime, and the time is short as it ...

4

The term "orbit" means that an object moves around a point in space on a certain path. Generally, this center point is an object - like your examples - or, in a binary system, a point in space called a barycenter, around which both bodies orbit. The barycenter is located at the system's center of mass. Commonly cited examples of orbiting objects are the ones ...

3

Generalizing the term "orbit" to mean some larger object / collection of objects to which the object in question is gravitationally bound, I'd say that the Milky Way "orbits" the Local Group, which in turn "orbits" the Virgo Supercluster. Beyond that, the expansion of the universe starts to dominate over gravitation. There are larger structures than ...

3

This is too long for a comment so I'll post it as an answer, even though this question is years old. If Alcubierre warp bubbles are physically possible, which is exceedingly unlikely, and if the equivalence principle is correct, you could definitely escape from a black hole in one, because there's nothing locally special about the event horizon. In a large ...

3

Alright, here's the skinny. The universe is much much larger than 92 billion light years. But the region we can see is around that number. Also, you have it all wrong for the expansion rate of the universe. The expansion is more like stretching. No matter where you are (that's an approximation), you would see a length x expand by the same amount after a time ...

3

I will select quotes from the wiki article on structure formation, bold mine: The very early Universe In this stage, some mechanism, such as cosmic inflation, is responsible for establishing the initial conditions of the Universe: homogeneity, isotropy and flatness.3[6] Cosmic inflation also would have amplified minute quantum fluctuations ...

3

The relative speed between two objects is only restricted within the special theory of relativity. These restrictions are only guaranteed to apply in general relativity – the theory of curved space that you need for the Big Bang theory – if the space surrounding the objects is the flat Minkowski spacetime, or at least can be approximated by the flat ...

3

I will reply to Why isn't the CMB at the edges of the universe? Why is it flying around in the middle? The occurrence of space time and matter after the Big Bang happened to all points in our universe. The expansion of space happened at the same rate outwards for all points of the universe. All points of the universe 380.000 years ago had ...

3

If you take an isolated spherically symmetric object then the spacetime curvature around it is described by the Schwarzschild metric. The bending of the rubber sheet is meant to be an analogy for this curvature, but bear in mind it's just an analogy and is in many ways a poor representation of what actually happens. Anyhow, the Schwarzschild metric only ...

3

Galaxy rotation happens at a very slow rate (compared to the speed of light). Let's suppose you are observing a galaxy edge-on that the delay from the farthest point is $\Delta t = d/c$, where $d$ is the galaxy diameter. If we take the lag from one extreme point to the other as D: $D = vt = \frac{v}{c}t$ (where $v$ is the rotational speed). You can see ...

3

I think that "observable universe" is not defined precisely enough to make such statements about it. The spacetime events that we can see are the events on our past light cone. That light cone intersects the last-scattering surface (about 400,000 years after the big bang) in an approximate sphere. By convention the light cone is cut off there (because we ...

3

How a unit is chosen to be defined depends in large part on how precisely the unit can be reproduced based on that definition. Two different atomic clocks built using the best currently possible methods will produce almost exactly the same answer for how long a second is, to within about 1 part in $10^{14}$. The second is defined in terms of a property of ...

2

The argument is sound given a few oft-omitted (but not too unreasonable) assumptions. Here is one way it can be formulated. Consider a volume $V$. Suppose it has a (possibly infinite) set of possible configurations; call this set of states $S$. Suppose we are interested in a particular configuration, $c \in S$, to within a certain tolerance. Let $C ... 2 Galaxies would appear stretched along the line of sight, not jumbled. Let's say a galaxy is ten million light years away and, as you proposed, is 100,000 light years across and we see it nearly edge on. The front of the galaxy will appear to us as it did ten million years ago and the back of the galaxy as it did 10,100,000 years ago. Thus, if the galaxy ... 2 The current entropy in the Universe is all stored in photons. The first reference by Qmechanic gives you the precise value. Since the photons of the CMBR do not at present interact with anything, the entropy of the Universe is very close to being a constant. What evolution there is, is all due to non-reversible processes in baryonic matter, but it amounts to ... 2 The question is what do we need the matter content of the universe for. As I understand it, in the usual case we want to find the conserved quantity associated with a certain conserved current gained by the projection of the energy-momentum tensor into a Killing vector, as for example in the paper by Abott and Deser. The requirement of asymptotical ... 2 On a plot of$\Omega_\Lambda$versus$\Omega_M$, there are three sets of observations that provide constraints: supernovae, the cosmic microwave background, and baryon acoustic oscillations. These three regions in the$\Omega_\Lambda$-$\Omega_M\$ plane all have a common region of intersection, which is quite small. If they had failed to overlap, it would have ...

2

Consider the way the Earth is illuminated by the Sun: Light travels in straight lines, so on the Earth we only get the light that happens to be travelling in our direction, and of course this is a tiny fraction of all the light emitted by the Sun. Now suppose we could place a gigantic lens in between the Sun and the Earth to focus the Sun's light onto ...

2

Classically, the CMB radiation would never completely disappear. It would just become more and more faint and redshifted. Quantum mechanically, if ΛCDM cosmology is correct, only finitely many CMB photons will reach us, so there will be a last photon. (edit: In general big bang cosmologies, there may be no last photon. I formerly said that the photon ...

1

We do not know if the universe is closed or open, so space could very well be infinite. However, that does not mean that there is an infinite amount of space in anything. Such a conclusion does not quite make much sense in terms of a logical,mathematical (or even philosophical) argument. Take Zeno's paradox for instance: The paradox states(in summary) that ...

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