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Can someone please explain what exactly constitutes to our conclusion of an expanding universe? I know it involves looking at distant galaxies and measuring the red shift, but can someone just clarify exactly what that involves and why the conclusion is drawn?


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I am just kinda obsessed with theories about apocalypses and all and just today I found an image of a black hole sucking a gigantic star, which really terrifies me about the outlook where we all doom just because a black holde decides to pay us a small visit and somehow finds us a good choice for his brunch. I don't even know the purpose of me posting this, it is not even a question but if there's anything you know about this topic, like is there really that chance of the Earth being sucked by a black hole someday, will modern science predict that fast enough and be able to find a way for human to evacuate or avoid the black hole or such? any other interesting information is appreciated too thanks in advance!


I’m reading Steven hawking book, the universe in a nutshell, and am at the part where he introduces time in the context of relativity. According to general relativity (and the great Steven) there was no time before space - they are inextricably linked. As we’ve relatively recently learned the universes is not only expanding, but expanding at an increasing rate. This got me thinking - what, if any, effect does this have on time?

Posted byu/[deleted]6 days ago

I’ve had this question for a while because it would seem to my stupid brain that if c would in fact not be the universal speed limit and instead be infinitesimally smaller, that would solve a paradox or two.

How do we know for sure that light travels at the universally highest speed. Does gravitational lensing not imply that light has mass and therefore shouldn’t be able to go as fast as universally possible? I hope my question makes sense.

Crossposted by8 days ago

June 11, 2018 is the tenth anniversary of the launch of NASA's Fermi Gamma-Ray Space Telescope. Over the last decade, Fermi has been a powerful discovery machine, providing insight into a wide variety of different astrophysical sources and extreme events. We've detected gravitational wave-producing merging neutron stars in a distant galaxy, found antimatter coming from thunderstorms, and discovered a huge, previously unknown structure in our galaxy now known as the "Fermi Bubbles.”

In addition, Fermi has partnered with numerous artists over the course of the mission, including composers, sculptors, and animation students who create amazing works of art inspired by Fermi’s gamma-ray science.

During our AMA, you can ask Fermi scientists and creators your questions about this amazing mission, the discoveries it has made, and the art it has inspired. Keep up with news from the mission at and find out about Fermi's 10th Anniversary activities at


We are:

Mattia diMauro (MdM) | Astrophysicist, Stanford University

Elizabeth Ferrara (ECF) | Astrophysicist, University of Maryland

Jay Friedlander (JF) | NASA graphic artist, NASA Goddard Space Flight Center

Liz Hays (EAH) | Fermi Deputy Project Scientist, NASA Goddard Space Flight Center

Michelle Hui (MH) | Astrophysicist, NASA Marshall Space Flight Center

Tyrel Johnson (TJ) | Research Associate Professor, George Mason University

Daniel Kocevski (DK) | Astrophysicist, NASA Marshall Space Flight Center

Nestor Mirabal (NM) | Astrophysicist NASA Goddard Space Flight Center

Roopesh Ojha (RO) | Astrophysicist NASA Goddard Space Flight Center

Judy Racusin (JLR) | Fermi Deputy Project Scientist, NASA Goddard Space Flight Center

Oliver Roberts (OJR) | Astrophysicist, NASA Marshall Space Flight Center

Shoshana Schlaudefaff (SS) | MICA animation student, NASA Goddard Space Flight Center

Dave Thompson (DJT) | Fermi Deputy Project Scientist, NASA Goddard Space Flight Center

Giacomo Vianello (GV) | Astrophysicist, Stanford University

246 points

What solution would have the most profound impact on the scientific community? Or even the public at large?


1 second after the Big Bang, the Universe was about 20 light years in diameter. So how large was it after 1 year?


I don’t have a coherent understanding of the basic inflation-based multiverse and how it fits with the theory of the big bang, including inflation.

My general image of the big bang is that if you take the Friedmann equations, which describe the expansion of the universe based on the theory of relativity, and run them backwards in time from the present, you get to a time, which we call t = 0, where there is a singularity in the equations due to the infinities of density and mass. But before this tine t = 0, where the equations stop being meaningful mathematically, we reach a time, about 0.6 * Planck time, where they stop being meaningfully physically because the massive densities make relativity not an accurate model. Unfortunately, we do not have a complete quantum theory that allows us to look beyond this point. So, the big bang theory essentially begins at about 0.6 * Planck time with a very dense universe that is expanding at every point in the universe. In order to explain certain uniformities in the visible universe, it has been proposed that there was a very short period of massively larger inflation.

As I understand it, the multiverse theory comes from the idea that this larger inflation did not end every were in the universe. This view pictures a universe that is highly inflating at most points but with occasional points where inflation ends. This corresponds to the creation of a sub-universe, and corresponds to a big bang for that sub-universe.

So here are a couple of the things that don’t fit for me.

How does the stopping of inflation at a point look anything like the big bang? A point in space where a highly inflating universe suddenly reduces inflating, would seem unlikely to be dense.

Secondly, our universe does not seem to fit with the idea of being a bounded sub-universe within a background multiverse where we are separated by vast amounts of inflating space from other sub-universes. Assuming that it was one point that dropped out of inflation, then our current universe would be approximately a Hubble volume, with us closer to a boundary than the center. So there should be more stars in one direction than the other. Perhaps, a vast region of inflating space dropped out of inflation to create our universe, and we are in a Hubble volume away from the boundary?

Any help understanding this would be appreciated.


How can an object fall into a black hole if the black hole will theoretically evaporate before the object falls in due to time dilation and hawking radiation?


hi all, I have a question about cosmology that I was debating.

I was wondering if the measurements (like supernova observations) would rule out the possibility that the universe's expansion is parabolic. I'm not really asking whether it would be a possibility from a theoretical point of view, just whether the supernova / CMB / whatever data could support a parabolic universe?

I have an understanding that we have a pretty good idea of how the universe expands based on energy pressure / the reasoning behind the Friedmann equations.

What I don't have, is a good understanding of whether or not the current experimental evidence would, at least in principle, be consistent with a parabolic universe that started with a very small scale factor, and grew parabolically:

a(t) = a_0 t^2 / current time^2 + epsilon


I understand how a black hole can be formed, by being crushed down to its Schwarzschild radius.

Examples would involve crushing the earth down to the size of a peanut, or crushing the sun down to the size of a small town.

But what about super massive black holes, that apparently even dwarf the size of our entire solar system? Were these black holes much bigger objects before they became black holes (that were crushed down to their S radius) or were they smaller black holes initially that just absorbed mass as time went on?


To my knowledge, circumnavigating Solar explorations have always pointed "inwards" towards the sun, with the goal of mapping it.

What if something of the same mass was at the nadir of our orbit around the sun. How could we tell? Would it not be beyond deductive calculation on celestial orbits, since we assume the net mass on planets is all our own?

In an attempt to straighten out my mental projection of orbital mechanics, the net effect of Solar orbits would be double, but what if we've been assuming a unitary orbit/mass all along.


Is it possible that a closed universe with k>0 transform into an open universe with k<0 ?


As some of you may know, the 4th dimension is time (1st is x, 2nd is y, 3rd is z and 4th is time) My question is what is the 5th dimension and do we even know what it could be?


In trying to understand redshift, I have a hypothetical question… If the universe is neither expanding nor contracting, but a distant galaxy appears to be moving away from us, is there still a redshift to its light? (I do understand that the universe is actually expanding. But for the purpose of this question, let’s take the hypothetical example of a universe that is neither expanding nor contracting).


I have seen before a video of scientists using cloth to explain gravity, but that was just simplified because spacetime fabric is actually three-dimensional and looks nothing like a piece of cloth, so I assume that black holes have a spherical shape and they are huge spheres containing something similar to anti matter. Am I correct?


It's a simple question: Since in our frame of reference, an object needs infinite time to reach the EH, and IF this apply to one black hole falling in to the other one (i'm guessing it does, but im not a GR expert) , how can two BHs merge? Wouldn't they need infinte time for their EHs to meet?

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