# Tag Info

8

Generally speaking they refer to the distances from us when the light as emitted. No correction is usually made to say how far away the object is from us now, because this correction would be very small and inconsequential compared to the uncertainty in the original distance measurement. For instance, taking the Andromeda M31 galaxy as an example. Riess et ...

7

In the weak field limit, which applies to all the cases you've described, the difference between the time rates for two observers with a Newtonian gravitational potential energy difference of $\Delta\Phi$ is given by: $$\frac{\Delta t_1}{\Delta t_2} = \sqrt{1 - \frac{2\Delta\Phi}{c^2}} \tag{1}$$ Note that the time dilation is related to the gravitational ...

6

Stellar motion would imply we're in the place where the Big Bang occurred while everything else is speeding away from us. I can see how this would be a worthwile explanation for a creationist, but it doesn't really make much sense building just on physics. Doppler Effect has the same issue - it assumes that every star we can see moves in a speed ...

6

Light always travels at the speed of light when in a vacuum. Space is a pretty good vacuum. So if it's been travelling for 13.7 billion years, then it has travelled 13.7 billion light years. There is no contradiction here. Yes, those galaxies are now 46 billion light years light years away, but this is because the universe has expanded. You can find lots ...

5

Fluctuations of density in the universe naturally become greater with time because matter is attracted to regions that are denser than average, and as a result they get denser still and the other regions less dense. So if there were even tiny density fluctuations in the early universe they would have grown into the density variations we see today - ...

5

Another line of thought is that there are other observations that support the notion of an expanding universe - it does not all hinge on redshift. For instance, I believe numbers 3 onwards would struggle to explain why the light curves of otherwise similar supernovae become stretched (at a variety of wavelengths) at larger distances from us and by an amount ...

5

It doesn't make sense to say "am I experiencing time dialation?" It only makes sense to compare two different observers, and ask whether one of them observes the others' clock to be ticking more slowly, say, when they are looking at minimum distance light rays coming from the other observer. With this in mind, the answers to most of your questions are ...

4

Gravitational redshift See this answer: http://physics.stackexchange.com/a/113941/4552 Photons interacting with something There are various problems with this, including problems explaining surface brightness. For more information, see http://en.wikipedia.org/wiki/Tired_light Transverse Doppler shift There are strict empirical limits on cosmological ...

4

There's (almost) no difference between matter and antimatter, and it would be very difficult to know if a galaxy far away is made of matter or not. The reason we know that the visible is made of matter, and not half-matter and half-antimatter is that it this were the case, then we would see a ton of events where anti-matter galaxies would smash into a ...

4

Neither the space between galaxies nor the space within galaxies is completely empty, but rather is filled with a diffuse gas. If there was an isolated galaxy, or region of galaxies that was made up of antimatter, there would be an interface between the two regions, and this region would be rich in annhilation events, which would be visible in X-Rays. We ...

4

First off, spherical symmetry isn't really the best description. Cosmological models usually assume that the universe is (approximately) homogeneous and isotropic. That's a higher degree of symmetry than spherical symmetry. Spherical symmetry would normally be used to describe something that has a lower degree of symmetry, so that there is a center. The ...

4

A quick note in the light of some of the comments: I'm interpreting the question to be asking about the escape velocity from a black hole containing a naked singularity rather than the escape velocity from the singularity itself. The escape velocity at the singularity is undefined as GR cannot describe the geometry at that point. Anyhow, a convenient way to ...

3

It's tempting to think of spacetime as a thing, and it doesn't help that it's often represented as a rubber sheet in popular science programmes. In relativity (special and general) spacetime is a mathematical concept - it is a manifold equipped with a metric. At the risk of over-simplifying, a manifold is a thing that has dimensionality (four dimensions for ...

3

What are their purpose? The "purposes" of Hyperbolic geometries are many and varied in mathematics, but one stands out far beyond all others, at least historically as the purpose. Hyperbolic geometries were constructed to prove that the Euclid parallel postulate (see "Parallel Postulate" Wiki page) was logically independent of Euclid's other axioms of ...

2

"Only about 10 percent of the total baryonic matter is sufficiently condensed by gravity to form stars and galaxies. More than 90 percent was left between the galaxies." http://hubblesite.org/hubble_discoveries/science_year_in_review/pdf/2008/searching_for_baryonic_matter_in_intergalactic_space.pdf 6% of baryonic matter is within stars according to the ...

2

The purpose of any model of the hyperbolic plane is that some aspect in it will be easy to work with computationally or intuitively, e.g. writing out certain isometries, identifying geodesics, computing volumes, etc. Hyperbolic space is unbounded, a hyperbolic manifold can be bounded. I don't know Yes If you are not using the terms bounded and infinite to ...

2

The singularity comes from the scale factor $a(t)$: $$ds^2 = -dt^2 + [a(t)]^2 ( dr^2 + r^2 d \Omega^2)$$ By solving the Friedmann equations for the scale factor we know that: $$a(t) = a_0 t^{\lambda}$$ where $\lambda$ is some positive number that depends on the matter-radiation ratio of the universe. At $t=0$ the scale factor becomes $a(0)=0$. So at ...

1

You need to include the 'zero' versions of $\rho$ and $a$ in order for the units to works out, or, to put it another way, to account for initial conditions. As regards the 32, you made an error while integrating: $\int a \mathrm{d}a = \frac{a^{2}}{2}$; the factor of 2 taken to the other side under the square root provides a 4 to makes the 8 into a 32.

1

In Zero-Energy Model, negative energy associated with Gravity counterbalances positive energy associated with matter, photons, etc. So, No, Big Bang wasn't cold. You are just looking at partial picture (you just ignored Gravity). This is what Zero-Energy Model says: With traditional Big Bang model (which doesn't contain Inflation), the universe started out ...

1

In the standard homogeneous cosmological models the total energy in an expanding volume is zero. This is true for positive, negative or zero curvature and it must take into account the gravitational energy (which is negative), dark energy, matter and heat. Since the gravitational energy is negative the heat can be positive and increasing as you go back ...

1

$$\dot{\rho}+3(\rho+P)\frac{\dot{a}}{a}=0$$ with $P=0$, leads to $$\dot{\rho}+3\rho\frac{\dot{a}}{a}=0$$ $$\dot{\rho}=-3\rho\frac{\dot{a}}{a}$$ $$\frac{\dot{\rho}}{\rho}=-3\frac{\dot{a}}{a}$$ $$\frac{\frac{d\rho}{dt}}{\rho}=-3\frac{\frac{da}{dt}}{a}$$ $$\frac{1}{\rho}\frac{d\rho}{dt}=-3\frac{1}{a}\frac{da}{dt}$$ ...

1

At this point in time, your questions are subject to research - we just don't know, yet. We see the effects of dark matter (in rotation curves of galaxies, in gravitational lensing and others), so there is "something". But what exactly that "something" constitutes is not clear. There are several theories, and to think of dark matter as some kind of ...

1

I could probably go along with this assertion; except that the observable universe is not infinite. We can only see light reaching us from about 13 billion light years away. The stars/galaxies etc that formed in those distant regions are now even further away from us, but nevertheless, the observable universe is finite. Expansion of the Universe, will light ...

1

The volume of the universe is infinite, therefore there must be an infinite number of worlds. But not all of them are populated; therefore only a finite number are. Any finite number divided by infinity is as close to zero as makes no odds, therefore we can state that the population of the Universe to zero, and anyone you have ever met is merely a figment ...

1

Which scientists have called dark matter and dark energy forces? Dark matter is a form of matter that doesn't feel any force other than gravity (so far as we have observed). It's true that dark matter bends light because it has mass and it makes up about $5/6$ of all the mass in the universe. The gravity from this mass is what bends light through ...

1

Einstein's equation is a mathematical model devised to provide an approximate description of the universe. We know it's approximate because it generically predicts singularities that we believe to be unphysical, and it takes no account of quantum mechanics. So the question is whether the equation:  R_{\mu \nu} -\frac{1}{2}R\,g_{\mu \nu} + \Lambda\,g_{\mu ...

1

Here is the scenario from wikipedia According to inflation theory, the inflaton is a scalar field that is responsible for cosmic inflation in the very early universe. A quantized particle for this field is expected, similar to other quantum fields, called an inflaton. The field provides a mechanism by which a period of rapid expansion from 10^−35 to ...

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