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23

Yes it's falsifiable and indeed has been falsified. The geometry of spacetime is described by an equation called the metric that we get by solving Einstein's equation. We can get some information about the metric that describes the universe by studying the motion of objects like galaxies, galaxy clusters etc. When we do this we find the observations are ...


8

The Big Bang Theory is a much more general and less specific description of our theory about the origin of the Universe than the $\Lambda{\rm CDM}$ model (by the way, I don't think that the hyphen is written in that acronym). The Big Bang Theory says that the Universe was expanding and the distances between two places where galaxies sit today used to be ...


5

The most straightforward theories of inflation assume there exists some scalar field $\phi$ that permeates the Universe and drives inflation. Over time this scalar field changes, and the rate of change is given by $\dot{\phi}$. There is also some "potential energy" associated with the scalar field, which is given by some function $V(\phi)$. The specific ...


4

One of ideas associated with string theory is the ekpyrotic universe. This starts with brane cosmology i.e. the idea that our universe is a four dimensional brane floating around in the ten dimensional string theory spacetime. There will be many such brane worlds and the ekpyrotic idea is that a collision between two branes would appear just like the Big ...


4

Conformal space is nice because in it, photons have straight world-lines, so we can easily see what we must do to achieve causal contact between two points in the CMB, after the physical time $t_i=0$ of the initial singularity, but before the physical time $t_{\text{CMB}}$ of decoupling. Since we have \begin{equation} d\tau=\frac{dt}{a(t)}, \end{equation} ...


4

Throughout the question I will use $p(T_1)$ and $p(T_2)$ to denote the 4-momentum of the baseball at times $T_1$ and $T_2$, $\mathbf{v}_1$ and $\mathbf{v}_2$ to represent the spatial component of its physical velocity, and $a(T_1)$ and $a(T_2)$ to represent the scale factor of the Universe at these times. The homogeneity and isotropy of the Universe mean ...


4

The cosmological redshift of a galaxy is not interpreted as being due to the velocity of that galaxy away from us (the special relativistic interpretation), but rather as being being due the effect of the expanding space on the traveling photon (the general-relativistic interpretation).$^\dagger$ This expansion in turn makes the galaxies recede from us at a ...


3

Let's analyse the evolution of the curvature in the $\Lambda\text{CDM}$ model. If $\rho_R$, $\rho_M$, and $\rho_\Lambda$ are the densities of radiation, matter and dark energy, and $$ \rho_c = \frac{3H^2}{8\pi G} $$ is the critical density, then we can define $$ \Omega_{R} = \frac{\rho_{R}}{\rho_{c}},\quad \Omega_{M} = \frac{\rho_{M}}{\rho_{c}},\quad ...


3

This is an active (hot?) topic of research, in fact I attended a workshop on the subject just last week. In brief, no one has found a dark matter (DM) halo yet that does not host a galaxy, though we would very very much like to! The first reason it's so difficult to find a DM halo that does not have a galaxy is that a common working definition of a galaxy ...


3

A massive object (such as a galaxy) along the line of sight to a distant bright source (such as a quasar) bends the light along its path. If the "lensing" object is massive enough and the geometry is right, the background object can be seen as multiple sources. For instance, here is a galaxy (central point) and four images of a single quasar: For a ...


2

The statement that at the beginning of the universe energy/mass was concentrated in a single region under conditions of extreme temperature and density is usually extrapolated from experimental data on the energy/mass content of the universe and its expansion, which is then analyzed through the classical (i.e. non-quantum) theories of general relativity and ...


2

Such measurements have been done, using lasers reflecting off mirrors on the moon. See e.g the paper Progress in Lunar Laser Ranging Tests of Relativistic Gravity (Williams et. al. 2008) which established an effective limit on the expansion at AU scales that is about 80 times smaller than what would be expected if cosmological expansion applied within our ...


2

It's impossible to draw an accurate picture of a 2D hyperbolic surface, because such a surface cannot be embedded into a 3D euclidean space; this is known as Hilbert's Theorem. The saddle surface in the figure is just an approximation, and serves as an illustration that every point on a hyperbolic surface is a saddle point.


2

The vast majority of the star like objects we see in the sky are stars in our own galaxy. Assuming the accelerated expansion is due to a cosmological constant, and assuming the value of the cosmological constant does change (it's currently of order $10^{-52}\,\text{m}^2$) the expansion will never be strong enough to disrupt the Milky Way. So our night sky is ...


2

No, we will always be able to see farther than the Hubble sphere (in theory). This spacetime diagram — taken from Pulsar's rendering of Davis & Lineweaver (2003)'s Figure 1, in this excellent answer — can help visualize it: Coordinates In this figure, time increases upward, we're the vertical line in the middle, Big Bang is the bottom line, and our ...


2

Until dark energy (and dark matter) are properly understood, it is impossible to be certain of the future fate of the universe. The concordance $\Lambda$CDM model, deduced from observations of distant supernovae, from the cosmic microwave background and from baryon acoustic oscillations suggest that the expansion of the universe is accelerating and that ...


2

Yes, it can. Curvature (whatever measure for it you use, Riemann tensor, Ricci tensor, Ricci scalar, you name it) is a function of spacetime, and hence of time.


2

A rotating reference frame is not an inertial reference frame: In the rotating frame, objects accelerate even though there are no forces acting on them. In your example, you can in fact determine easily whether you are rotating or the universe is rotating around you. In the first case there is artificial gravity on the ship, and in the second case there is ...


2

The sentence shouldn't be read as "[velocity of energy] forms", but "velocity of [energy forms]"$^\dagger$. The sentence refers to "energy forms", i.e. the different forms in which energy can manifest itself. These forms are e.g. dark matter, normal ("baryonic") matter, and radiation. So what the authors (and others) refer to as the cosmic rest frame is the ...


2

When calculating redshifts, we usually look for signature features in astronomical spectra, usually emission or absorption lines. For example, the universe contains lots of hydrogen. From quantum mechanics, we know that hydrogen has many different energy states which are fixed. This means it can only emit photons with a particular set of wavelengths (these ...


1

While cnosam's answer is completely correct, I don't know if it really solves your confusion. The key point is that, when a photon is emitted, it knows nothing about the current size of the Universe. It is emitted at a very specific wavelength given by quantum mechanics, not by cosmology. Traveling through expanding space subsequently increases its ...


1

The temperature is inversely proportional to the scale factor. If you're interested in the gory details see Cooling in the Universe by Sohrab Rahvar. So your question reduces to how the scale factor $a(t)$ has changed with time. There isn't a simple answer to this because there is no simple analytic function for $a(t)$. It has to be computed numerically ...


1

EDIT: if i got it right , the answer to my question has to do with space-expansion , but to be honest i can't understand why. In an hypothetical situation that universe stops expanding and starts shrinking, we won't be able to detect such radiations anymore? One has to think of our three dimensions we live in and the effect of expansion or ...


1

This is a very tough question, since you do not make any further assumptions about the force - most importantly its strength, sign, variation with distance and objects it acts upon. In general, yes, the universe would probably look absolutely different and we would not be here to ask this question. Somebody or something absolutely else might. Our ...


1

What is a simple explanation to the fact that the universe is expanding? Show them a star map and point out the galaxies and the fact that it is a projection of three dimensional space to an image. Astronomers and astrophysicists have spent a lot of effort measuring the behavior of the galaxies, their motions relative to each other and to the solar ...


1

Firstly, you must know that there are many models for inflation which give different results to your a) and b) questions, and we still don't know which is the right one. I'll try to answer regarding the most accepted and simple models. a) During the period of inflation the distance between two separated points in the Universe increased at least ...



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