Light Travel Time from Exploding Stars

Question:  I am not able to understand one thing, when we say we saw a star exploded billions of light years away, does it mean it exploded billions of light years ago but the light reached to you just now. Aren’t we studying the phenomena which has happened billions of light years ago. Is it possible that another big bang has already happened, just the explosion light/energy has not reached to us yet?  – Dipanshu

Answer:  When we say that we have seen a star explode that is billions of light years ago, it means that it exploded billions of years ago since that light from that stellar explosion had to travel billions of light years to get to us.  Note, though, that a stellar explosion is not the same as the big bang, in that the big bang expansion started everywhere at once, and was not characterized by an explosion at a single point.

Jeff Mangum

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The Kerfuffle Over Non-Cosmological Quasar Redshifts

Question:  Some years ago I read an article that said (if I recall correctly) that there were quasars that seemed to be associated with galaxies (maybe in the center), but the quasar’s much larger red shifts implied that their distance was far more than the associated galaxies’ distances.  Has this ever been resolved?  – Bill

Answer: The research that you are referring to was done mainly by two astronomers, Halton Arp and Geoffrey Burbidge.  They proposed, based on observations of seemingly associated nearby galaxies and purportedly distant quasars, that quasars were simply ejected matter from these galaxies.  In fact, once large surveys of galaxies (such as the Sloan Digital Sky Survey), became available it was possible to better test this apparent correlation.  In summary, Arp and Burbidge were wrong, their assertion due in fact to what astronomers call a “selection effect”.  If you are interested in more details on this now historical discussion see the Galactic Interactions blog post on the subject.

Jeff Mangum

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How Do Astronomers Study How Things Evolve?

Question:  We know that stars and galaxies we see are just fossil light as they were millions or billions of years ago. Is it possible to extrapolate the changes that we see today in those galaxies to determine their current state?  – Vinod

Answer: In a way, yes.  Since, as you point out, we see what amounts to the “fossil light” from stars and galaxies in the universe, we can piece-together how things evolve with time by sampling various times within this fossil record to study the evolution of these stars and galaxies.  Note also that the timescales for the evolution of objects in the universe are, with few exceptions, much longer than a human lifetime, or even the total historical record of scientific measurements.  This means that astronomers must study the evolution of just about every object in the universe by sampling its evolutionary state at different times in the cosmological record.

Jeff Mangum

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What is a Fairly Representative Radio Astronomy Project?

Question:  What is a fairly representative project a radio astronomer might be involved with perhaps to investigate dark energy or some other equally interesting phenomenon?  – Steve

Answer:  As the number of interesting projects that one can do with radio telescopes is too large to list, let me list a radio telescope project that investigates the early universe.  The Hydrogen Epoch of Reionization Array (HERA) is a radio telescope array that will, as the name implies, attempt to image the large-scale structure of the universe at the point when the first stars reionized the neutral intergalactic medium.  This aspect of the universe is one of the last unexplored phases of the evolution of our universe.

Jeff Mangum

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Dark Matter and the Stuff Humans are Made of

Question:  If dark matter does not emit or reflect any kind of light, (As I have heard from Neil deGrasse Tyson’s Cosmos series) then how come we can see [ourselves in] a mirror?  – Cyrus

Answer:  Scientists love to perform experiments that clear prove a fact.  One interpretation of this observation is that this demonstrates that we are definitely not made of dark matter!  So, simply by looking in a mirror you have proven something fundamental about the material that humans are made of.  Congratulations!

Jeff Mangum

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What is the Universe Expanding Into?

Question:  Thank you for helping so many people. Is there an educated guess by professionals about the difference between what the universe is expanding into and the universe itself?  – Jay

Answer:  This is going to sound very paradoxical, but the answer is that the universe is already infinitely big, so it is in fact not expanding anything.  What is actually happening is that the space between everything in the universe is getting stretched, which results is out seeing all galaxies in the universe, which are not under the influence of local gravity like a cluster of galaxies, moving away from all other galaxies.  Now, if you want a much more in-depth explanation of this rather odd fact, check out the Curious About Astronomy page answer to this question by my colleague Dave Rothstein.

Jeff Mangum

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Optical and Radio Measurements of the Gravitational Bending of Light Around the Sun

Question:  VLBA measurements have been used to confirm gravitational bending of radio waves by the Sun in confirmation of General relativity of 1″.75 prediction. Most optical observations are usually higher than this. What amount of VLBA data measurements show a 1″.97 bending by the Sun similar to the finding in the solar eclipse expedition of 1919 at Sobral, Brazil but which may have been edited out by software as being anomalous? – Akinbo

Answer:  Most scientists who have studied the experiments designed to test Einstein’s General Theory of Relativity believe that the optical observations, starting with the Eddington results from the observations made during the Sobral expedition, suffer from considerable uncertaintities.  The observations made with the VLBA, at radio wavelengths, are generally believed to be the most accurate.  They do, in fact, confirm Einstein’s prediction of the bending of light around the Sun.

Jeff Mangum

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Top Achievements for the NRAO Very Large Array

Question:  I’m embarrassed to ask this, as I feel after working here for so long I should already know the answer. But here it is: What are the top 3 discoveries/achievements of the VLA? And why?  — Doug

Answer:  As you can imagine, an actually ranking of the Very Large Array (VLA) achievements would be open to debate.  Kind of “beauty in the eye of the beholder” problem.  Dave Finley, NRAO Public Information Officer, provided the following links listing what NRAO believes are the VLA’s top scientific achievements:

Jeff Mangum

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Dark Energy, Universal Expansion, and Peculiar Galaxy Motions

Question:  I have been hearing lately in some space documentaries that black energy, which makes the universe expand and at an increasing speed, separate and repulse objects such as galaxies from one another, am I correct?  But i also heard that the Andromeda galaxy is heading straight for us and should merge with the milky way, isn’t it contradictory? Thank you in advance for taking the time. – David

Answer:  I suspect that what you mean by “black energy” is really Dark Energy, which is one of several explanations for the acceleration of the expansion of the Universe.  The folks at NASA have a very nice page which describes what we think Dark Energy is, which you should certainly check out.  You are correct that Dark Energy is theorized to act like a repulsive force which pushes galaxies apart.  As for the Andromeda galaxy’s peculiar motion toward us, this is due to the mutual gravitational pull between our galaxy and Andromeda.  This is a “small-scale” affect, while the overall expansion of the Universe happens on a larger scale.

Jeff Mangum


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Will Dark Energy Tear Apart Black Holes?

Question:  Will dark energy eventually tear apart black holes, especially supermassive black holes?  – Reinaldo

Answer:  Let’s start with a basic understanding what what Dark Energy is thought to be.  It is a hypothetical form of energy which is thought to pervade the universe.  It has been proposed as the source of the measured acceleration of the expansion of our universe.  As a physical entity it is really quite “thin”, having a density of about 10^(-29) grams per cubic centimeter, so its affect of things like planets, stars, humans, and black holes is exceedingly small.  But, because it is thought to fill the universe its affect on the expansion of the universe is very important.  So, to answer your question, Dark Energy does not affect the properties of black holes, so in fact cannot disrupt them in any way.

Jeff Mangum

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