Horizon Definition and 446 Threads

As I understand it, the statement of the horizon problem assumes that the uniformity of the CBR measured at opposite directions in the sky needs a mechanism to create this uniformity. I also understand that many cosmologists do not share this assumption. The purpose of this thread is to seek...

I am clearly talking about black holes. The event horizon is the limit where even a photon won't escape it. I tried to calculate it in the easy way using enegry calculation m * MG/R = mc^2 / 2 but I do not know if I am using the right equation or even if I can divide by the m because it...

Given enough time Boltzmann freak structures will appear, assembled from drifting matter, in the maximum-entropy universe if it is static, I.E. not expanding to eventually sweep all matter into the far horizon. In a lecture...

What's the best way to explain why tidal forces for an observer free-falling through an event horizon are finite? My first thought was to say that "gravity isn't a force, it's a curved space-time". On further thought, however, it seems to me that consideration of the Rindler horizon shows...

Dear PF Forum, Can we avoid spaghettification for some times, once we're inside event horizon? I choose a rather massive black hole, so the tidal force won't be so big at EH. Mass: 1 trillion solar mass Schwarzschild radius: 2.950 trillion KM What is the gravitational force at EH? I calculate...

Hello everyone, Questions keep on rushing through my mind. If we can see light from 300,000 years after the big bang (THE OLDEST LIGHT EVER) then why can't we see beyond our cosmic horizon? We are seeing the oldest light ever... then why do we have a cosmic horizon?

Can one create a black hole when already inside the black hole ?

One aspect of the Horizon Problem is that the observed equal CBR temperature from opposite directions of the sky is a mystery. If we choose two points, say A and B, at opposite ends of a arbitrary diameter of the observable universe, at the present time the distance between A and B would be D...

I have read that airplane pilots can first perceive the curvature of the Earth's horizon at an altitude of about 35,000 ft. (The horizon would be 230 miles away.) Precisely, what is the shape they see? I don't think it's an arc of a circle because of the angle of observation. Is it part of an...

Homework Statement I am preparing a report on black holes and I recently learned about a phenomenon I was previously unaware of: the photon sphere of a black hole. While reading an article on said occurrence (I have now confirmed this on multiple sources) the photon sphere which is the minimum...

The problem with the notion of Event Horizon is that it speaks ot events which will never be seen in the future. So it requires to wait eternally. So apparent Horizon is introduced. It uses http://www.fysik.su.se/~ingemar/relteori/Emmaslic.pdf They are closed spacelike surfaces. when they emit...

This paper; http://arxiv.org/abs/1506.08573, First law of thermodynamics for dynamical apparent horizons and the entropy of Friedmann universes, offers a themodynamic evaluation of the apparent horizon of the observable universe. While not for the mathematically faint of heart, it is interesting...

In classical mechanics, to raise from some height h_{0} to infinity over a gravitational body, takes a certain amount of energy, the energy associated with escape velocity, let's just call it ε. ε = \lim_{t\rightarrow +\infty} \int_{h_{0}}^t ƒ(h)dh Likewise, it's time-reversible, so dropping...

I've been working through Leonard Susskind's "The Theoretical Minimum" lecture series (which are a fantastic introduction to the topics covered by the way) and a couple of his comments confused me when he was covering the Kruskal-Szekeres metric/coordinates in General Relativity. The end of the...

Let's say I keep on dropping electrons on one spot of a Rindler horizon. Does the charge of the spot increase without a limit? When the charge of the spot is very large, does the spot exert a Coulomb force on the electron I'm about to drop, causing the electron to start moving away from the...

Forgive my ignorance here, I'm not a physicist, but the exact nature of gravity beyond the Event Horizon has me a bit stumped. Say I cross the Horizon of a Supermassive Black Hole, meaning the pull on me isn't really that much stronger than Earth's pull initially - why can't I then escape...

So the event horizon is the distance from the singularity where escape velocity exceeds the speed of light, right? Well let's say I am standing next to the event horizon, RIGHT next to it. What if I stick my arm in past the event horizon, then pull it out. I can't do that, right? Why not...

SR implies that mass can't reach c, but mass does reach a black hole's event horizon. How is this reconciled with SR?

According to Wikipedia, the gravitational time dilation formula is given by t_0 = t_f \sqrt{1 - \frac{2GM}{rc^2}} = t_f \sqrt{1 - \frac{r_0}{r}} where t0 is the proper time between events A and B for a slow-ticking observer within the gravitational field, tf is the coordinate time between...

Homework Statement A sub-atomic particle is near the event horizon of a black hole. Due to the nearby gravitational field, the Ricci Curvature Tensor is changing rapidly. The particle then performs quantum tunneling. Homework Equations Which version of spacetime does the tunneling particle...

If particle horizon is the maximum distance we can see presently in the universe, how come we are able to see CMB? CMB is radiation from surface of last scattering happened at t~380k years. Suppose universe is expanding at a constant rate ( i.e. no acceleration), will we be able to see CMB...

Hello, I remember hearing somewhere that it is possible to calculate the event horizon, I was just wondering if this is possible and what equations are used to calculate it.

I am still confused by the apparent contradictions between the world line of an immortal Observer at a distance (~100 M km) from a collapsing stellar core (-> Black Hole) and the reworking of coordinates to include the Schwarzschild radius. The argument that an in-falling astronaut would pass...

Suppose I extend a tether into a BH horizon. My spaceship is on the outside and my friend is on the end of a very long tether and crosses the event horizon...can I pull him back?

I am confused about black hole horizons and such common statements as "light cannot escape from inside the horizon". The way I currently understand it is as follows : 1. Horizons are always relative to an observer, and what is called "the black hole horizon" is just a shorthand for "the black...

Hi there, I have a metric with g_{rr}=\frac{1}{r^2-2cr}. From this it is clear there exist coordinate singularities at r=0 and r=2c. I believe that the outer horizon is the event horizon and the inner horizon is a Cauchy horizon. However, I do not know what I need to do in order to prove that...

When the apparent horizon differs from the event horizon, as in the case of an observer falling into a black hole, does Hawking radiation take place at the former, the latter, or both?

I'm quite aware of how to compute how FAR you are from the horizon, but my question is, how WIDE is the observable horizon at sea level (like, from left to right, how many kilometers is this): http://www.jeicentral.com/wp-content/uploads/2014/10/far_sunset_in_te_ocean_horizon-wide.jpg Thanks!

No one seems to be bothered by this except me: Black holes have a finite lifetime measured in Schwartzchild time due to Hawking radiation. Similarly, the universe probably has a finite lifetime measured in Schwartzchild time. In that case, nothing ever falls through the event horizon of a black...

Is there a physical boundary that is the event horizon? Or is there not? The reason I'm asking is because texts say that the event horizon that appears in Schwarzschild's metric is a result of the coordinate choice, and it disappears by choosing some other coordinates.

Hi! It is stated in V. Mukhanov's book "Physical foundations of Cosmology" the following (page 44, after equation 2.25): "In contrast, for the dust dominated universe, where ηmax=2π, the event horizon exists only during the contraction phase when η>π." could someone please explain why is this...

Hi guys, I have a GR question. It is usually said that black holes have event horizons in which time freezes/stops relative to an outside observer. This happens in the Schwarzschild coordinate system. But are there any coordinate systems in which the coordinate time of the black hole and its...

I'm having some trouble understand this correctly, so I was hoping someone could enlighten me a bit :) Okay, so in the early Universe most of the hydrogen and helium was formed, and then kept in equilibrium, and ionized via photons. So we have a plasma of baryonic matter, including dark matter...

Hello, Onhttp://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html website it says in the section "below the rocket, something strange is happening" that the distance of an object which passes the accelerating observer never increases -c2/α. I think this means, that the Rindler Horizon...

Stephen hawking came to know of a study that stated that surface area of an event horizon increases. So he said that since area has increased Entropy also had increased. But why does surface area of an Event Horizon increase?

So in 2009 there was this paper which described what was known back then based on observational data and the final sentence in the conclusion read as follows: Now for you astrophysicists here, has anything changed? It is my understanding, and please correct me if I'm wrong, that as of 2013...

I am having some problems with understanding what it means to be in the horizon of something else. In particular I'm looking into axion cosmology model. In this paper, and in particular in the strings and wall decay sections: http://arxiv.org/abs/astro-ph/0610440 Let me try to elaborate some of...

The event horizon is a null surface in spacetime separating events that are causally connected to an observer from those that are not. Does anyone have a good, simple argument on why it must be a null surface?

Are galaxies that exist beyond the horizon of the visible universe in a superposition of many different states relative to us? A related question: If we take a laboratory with some scientists inside and put them all in a big box that's perfectly isolated from the rest of the universe, then the...

My question is: Is my picture of what happens as something approaches an event horizon accurate - and are there any citations describing it? I got my "picture" from a PBS broadcast several years ago. It was since mentioned in these forums, but I haven't been able to find an explicit mention...

As all of us know that in a vacuum the particle antiparticle pairs or the virtual particles are created violating the law of conservation of energy. So even near an Event Horizon of a Black Hole virtual particles are formed. One of the particles gets sucked into the black hole but the other...

The ΛEPRL spin foam model presented 25 November at ILQGS by Haggard and Riello achieves an interesting quantization of the cosmological constant. Basically this is done on slide #10 around minute 15 of the audio. http://relativity.phys.lsu.edu/ilqgs/haggardriello112514.pdf...

If one of two entangled particles passes the event horizon of a black hole, will the entanglement still exist? Because that would mean information can come from a black hole.

Greetings. For my own edification I calculated a set of the congruence of uniformly accelerated observers in flat spacetime in the spherical polar chart. These observers accelerate radially outwards from some ##r## so that their horizons are at the same position ##r_h##. This requires that...

A number of recent threads have discussed what happens when an observer falling into a massive black hole passes the event horizon. What I would like to know is this. For a massive BH of mass M, Schwartzchild Radius Rs, how long would it take for such an observer (who, presumably crosses the...

Does anybody understand the fundamentals of what is happening in this circuit? Tungsten welding info is all about high current... but this is a low current scenario. AC input, CDI Coil generated 20Kv (estimated) suffers voltage drop over 100mm of tungsten rod. The larger the diameter... the...

I recently read a few articles that contradict Einstein's Singularity theorem. The idea being that black holes are wormholes to other universes; with a white hole on the other side of the black hole (Poplawski's theory). What if instead of being a portal to another universe, the Event Horizon of...

Homework Statement How do I calculate the temperature at which a galactic scale perturbation enters the horizon? This would be for radiation domination. Homework Equations \left( \frac{\delta \rho}{\rho} \right)_{\lambda_0} (t) = \left( \frac{a(t)}{a_{eq}} \right) \left( \frac{\delta...

Dear all, In one of his lectures,Prof. Susskind mentioned that the event horizon "bulges" forward to meet any incoming radiation or matter; and it is a property of Einstein field equations. I have not come across any such property, and if it exists, shouldn't it belong to the Schwarzschild(or...

I'm trying to understand the ideas in this paper at a nontechnical level: Laura Mersini-Houghton, "Backreaction of Hawking Radiation on a Gravitationally Collapsing Star I: Black Holes?," http://arxiv.org/abs/1406.1525 She says: This work investigates the backreaction of Hawking radiation on...