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12

The answer is yes time dilation does affect how much time an observer experiences since the big bang until the present (cosmological) time. However there is a certain set of special observers called comoving observers, these are the observers to which the Universe appears isotropic to. For example we can tell the Earth is moving at about 350 km/s relative ...


10

As Walter says, gravity doesn't bend light. Light travels along null geodesics, a particular type of straight path. Since (affine) geodesics don't change direction by definition, geometrically light trajectories are straight. Moreover, the speed of light in vacuum is $c$ in every inertial frame, regardless of whether or not spacetime is curved, although a ...


9

It is correct that the Kerr black hole solution of GTR allows travel between universes. However, that does not mean that if you actually jump into any kind of black hole that you could go to another universe. To motivate the resolution to this conundrum, let's start off very easy: suppose you stand on the ground with a ball in your hand, and you throw it ...


9

Gravity doesn't affect the speed of light. It affects the space-time geometry and hence the paths of light. However, this can have a similar effect. Light emitted at source $S$ to pass a massive object $M$ that is very close on the otherwise (if M weren't there) straight path to an observer $O$ has to "go around" $M$, which takes longer than following the ...


8

This has been considered long ago (Here's a paper talking about this). Wormholes are not forbidden by physics, but the creation of wormholes is iffy ground. THere are two possible paths one can take to create a wormhole: Choose a pre-existing wormhole in the quantum foam and "expand" it by feeding it exotic matter. "Tear and sew up" space — we're ...


8

No more than the observation of light waves disproves quantum mechanics. Light has properties of both a particle and a wave. At low energies, the particle nature of light is hard to detect: radio waves are made of photons, but individual radio wave photons are pretty hard to detect. I'm not sure that we have directly detected individual photons with ...


8

Another question, how can we identify the ripple's origin (let's say that if it's the result from the big bang or another big event)? (I'm just answering this part of the question, as James has already answered the main part about GR vs QM.) LIGO have produced an image which shows their best estimate of where these two black holes were: All they can ...


7

I'm not sure what you mean by saying that quantum gravity "doesn't exist". But because this is the Astronomy SE, I will interpret your question as primarily asking why astronomy hasn't found evidence of quantum gravity. This is a reasonable question; after all, nineteenth-century astronomers have found evidence of funny business in the perihelion precession ...


6

Newtonian gravity of a point-source can described by a potential $\Phi = -\mu/r$. If we suppress one spatial dimension and use it to graph the value of this potential instead, we get something that looks very close to this illustration, and is indeed infinitely deep at the center--at least, in the idealization of a point-mass. And farther away from the ...


6

Stellar clusters around supermassive black holes are systems in which relativity likely plays a role. Currently, only bright stars can be seen in our own galactic center because there is a ton of neutral gas between us and the galactic center that obscures it. As a result, we only have a few "test particles" out of the many stars that actually orbit the ...


5

This book by Andrew Liddle is fantastic for explaining the basic concepts and giving the reader a good mental picture of the ideas, but it may be a little more basic than what you're looking for (I still keep it as a good reference when reading more advanced texts). If you're looking for something more advanced Peter Coles & Francesco Lucchins guide ...


5

My topological defect cosmology is a little rusty, but I'm pretty sure this is how it goes. Start with the fluid equation, $$ \dot{\rho} + 3 {\dot{a} \over a} \left( \rho + p \right) = 0, $$ and the equation of state, $$ p = w \rho. $$ Plug the equation of state into the fluid equation, assume a constant $w$, and you'll find $$ \rho \propto a^{-3(1 + w)}. $$ ...


5

In the standard model, the universe looks the same for all locations moving in the local rest frame. This includes its apparent age. You can tell if you are in the local rest frame if the expansion of galaxies around you is symmetric in all directions and the microwave background also is the same in all directions. Simply put, any civilization on any ...


5

Orbits in Schwarzschild spacetime can be described by the effective potential $$V_\text{eff} = -\frac{GM}{r} + \frac{\mathfrak{l}^2}{2r^2} - \frac{GM\mathfrak{l}^2}{c^2r^3}\text{,}$$ where $\mathfrak{l} = r^2\dot{\phi}$ is the specific angular momentum of the orbit, which is a conserved quantity. The first two terms match the form of the Newtonian effective ...


5

Cherenkov radiation (Wikipedia) Cherenkov radiation, also known as Vavilov–Cherenkov radiation,[a] is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity of light in that medium. The characteristic blue glow of an underwater nuclear reactor is due to ...


5

If I travelled near a black hole, my time would progress slower relative to someone on Earth. This is clear enough. Yes, no problem with the gravitational time dilation. However, what if we sent a probe with a camera to a black hole? When we watch the screen, would we see time through the camera's perspective — that is, would the Universe appear ...


5

The impact of this measurement on the status of quantum gravitation is exactly zero. The proper statement of the incompatibility of general relativity and quantum mechanics is that the quantum field theory of general relativity is not renormalizable. Renormalizability essentially means that the theory is well-defined at all energy scales, which seems like a ...


4

You have many questions. I only answer the first. It's doesn't only matter how heavy a star is, but also how big. For ordinary stars, the effect is neglible (work it out by yourself -- it's a useful exercise). Even for compact stars, such as white dwarves or neutron stars, the effect is small. However, what astronomers commonly refer to as (stellar-mass) ...


4

I think the image you posted is not quite reallistic. On it, objects are just inverted from some radius on, while what you can expect from a real black hole seen from near enough is a combination of these: a) an accretion disc b) a companion being sucked c) Hawking's radiation d) X-Ray burst from the poles (really starting out of the event horizon) You ...


4

I will make a small calculation here, but please proceed to the results if you may like to. Calculation Stars are spherical and static, so metric near their surface (photosphere) and outside on is Schwarzschild. Hence time-time metric component on the surface is: $$g_{44}=1-\dfrac{R_{grav,*}}{R_*}$$, where $R_*$ is the radius of the star and $R_{grav,*}$ ...


4

"Allowed for" does not mean "necessarily cause". What the professor implied is that the solutions look, from a mathematical p.o.v., just like what you would expect from a bridge between universes - IF multiple universes exist, and IF the bridge is passable. That's all there is to it. A mathematical solution that looks like a bridge. But has it ever been ...


4

I don't know any specific text but I can recommend you a course I took last year at Coursera.org. It says "introduction" but I can assure you it is really complete, 12 week that include a lot of mathematics and physics. https://www.coursera.org/course/introastro


4

Let's restrict to special relativity, meaning two inertial frames moving in a Minkowski spacetime. A clock in the first inertial frame ticks slower, when seen from the second. A clock in the second inertial frames ticks slower, when seen from the first. Now assume, that you are fixed to one of the two inertial frames. Usually you measure velocities within ...


4

While the Sun and Earth attract each other, they cannot fall into each other because of angular momentum conservation. In a central field (where the force is acts in the direction of the distance vector and depends on distance only), the specific angular momentum vector $\boldsymbol{L}=\boldsymbol{r}\times\boldsymbol{v}$ is conserved ($\boldsymbol{r}$ is ...


4

There is evidence that both black holes and their event horizons exist. The primary evidence for stellar-mass black holes arises from observations of the dynamics of binary systems. What has been found, for at least 20 binary systems, is that the optically visible star has a dark companion, that is usually more massive, and more massive than can possibly be ...


4

If you're asking whether it's sufficient to use a retarded (time-delayed) positions to calculate gravitational forces, then no, that would be much worse than Newtonian gravity. For example, that would predict that the Earth should spiral into the Sun on the order of about 400 years. See also this question. Most small-scale N-body simulations (e.g., ...


4

A cosmological constant should be considered a special case of dark energy. The effective stress-energy tensor for a cosmological constant is proportional to the metric $g_{\mu\nu}$, so in a local inertial frame will be proportional $\mathrm{diag}(-1,+1,+1,+1)$. This is equivalent to perfect fluid with energy density and pressure directly opposite one ...


3

There are a few different features of black holes that researchers look for: An accretion disk Mass transfer from companion star X-ray emission Absorption of gas and other matter around it Gravitational lensing You mentioned #3 in your question; quite a few black holes, including Cygnus X-1, emit X-rays. 1 is a common feature around black holes. The ...


3

One of the predictions of General Relativity is that certain objects can give off energy in the form of gravitational radiation. This means that over time, the orbit of the two neutron stars should "decay", and they should come closer to each other. Using General Relativity, it is possible to predict the changes in the orbits and the energy of the emitted ...


3

That's a lot of not quite trivial questions! I'll try to answer part of it. First, the red-shift can be composed of relativistic Doppler-effect and gravitational red-shift. When neglecting the gravitationl part, we get a higher radial velocity. The radial velocity can be used to calculate a distance estimate via the Hubble "constant". So for low velocities ...



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