Question about Space Time and the Light we see from distant stars:

I came across this post at the NPR Science Friday message board about three weeks ago and it has been bugging me ever since. Essentially, the questioner is asking the following:

  1. Scientists often say that the light we see from distant starts is up to 13 Billion years old we are in essence they say “looking back in time.”

  2. The Universe is expanding (indeed astronomers say that the rate of expansion is accelerating).

  3. The matter in the Universe (including presumably our planet and Sun) move or (expand) much slower than light.

  4. Why didn’t that thirteen billion year old LIGHT from “distant” stars pass us long ago since when it started its journey we were much closer (since the universe had not been expanding as long).

  5. Therefore, the light we see now can’t be 13 billion years old. Unless, matter is expanding close to the speed of light (since it would take longer for the light to CATCH us).

This question is what prompted me to visit and then join this board. Thanks.
Message Subject: march 12th hubble ultra deep field
Posted By: physics101
Date: Wednesday, March 31 at 1:12 p.m.

on the march 12th Hubble Ultra Deep Field segment a question was asked by a caller, which is something I have pondered and have never had successfully answered. The scientist on the show obviously didn’t understand the caller’s question. He wanted to know how it is possible to see light from stars that are 13.2 or so billion light years away. If we started at the same point and matter can’t travel the speed of light how has the initial light of stars 13.2 billion years ago not passed us. We would have to be very near where we are now 200,000,000 years after the big bang in order for light that old to reach us and not already have passed us. Unless we started inflation near the speed of light and the net speed of light is so low that it could take that long, however, there would still be a limit. In addition wouldn’t that make the “Big Rip” theory impossible. We would already be breaking up. Since the universe is accelerating still and in order to see the light of a star 13.2 billion light years away, and space is inflating at close to the speed of light, then the stars must be nearly 26.4 billion light years away now, or they were 6.6 billion light years away when the light was emitted and scientists are just doing the math and the star is 13.2 billion light years away, but the light was emitted 6.6 billion years ago and we guestimate the distance the star would be now. the star has traveled immense distances beyond 13.2 billion light years away since the light we are now seeing from it was emitted. What makes this more confusing is how can we see light or CMBR 300,000 years after the big bang. We would have to almost instantaneously reposition to where we are now, or be traveling (inflating)over 99% the speed of light. This also applies even if the earth and the other object our heading away from eachother netting a speed close to the speed of light. Sorry for being so verbose. **

It doesn’t matter where the star is NOW. What matters is where the star was when the light was emitted.

That’s the thing when the star emitted the light it would have been MUCH closer to our position then (or where our position would ultimately be since the Earth/Sun were not even formed 13 billion years ago). Since, that LIGHT from 13 billion years ago should have LONG passed our position. Thus, the light we are seeing now might be considerably younger. Unless of course the rate of expansion (matter included) in the universe APPROACHES the speed of light (in which case it might take 13 Billion years to “overtake” our position even if our relative positions were relatively close when the lights journey began).

It’s believed that the initial expansion of the universe was very, very fast, nearly the speed of light. Stars didn’t form until a little later, as gravity began to slow things down and cause stars and galaxies to form. By that time, the expansion had slowed quite a bit.

Because (most likely) the universe was still friggin huge 13 billion years ago–it was smaller than it is now, but still had a radius of at least 13 billion light years. So the light we see is not only old, but from far away, anyway.

We don’t see old light that was emitted from the local neighborhood. That’s gone. Aliens on planets 5 billion light years away are just now seeing light that our sun emitted when it was born.

was established very early BEFORE the photons in question began their journey. Thus, the expansion/inflation that has occured SINCE is almost inconsequntial relative to the initial distance? One of the astronomers at the other site (Science Friday) doesn’t subscribe to expansion/inflation. Speaking of inflation have scientists compared Red Shift values taken recently from those taken say thirty years ago (from the same stars). This shoud allow us to calculate how fast/far our universe is expanding. I also seem to recall a trigometric technique whereby if you KNOW the distance of two points (say satellites on opposite sides of the Earth in orbit) AND the angle between them to a THIRD point (read a far away star) that you can calculate the distance to that third point. Has this been done and the values obtained compared to the “Red Shift” computed values?

Also, while I’m on the subject if our SUN/planet were at the leading edge of the expanding universe (I guess meaning that we were much older) and we put a space telescope like Hubble in orbit could we point it “outwards” into the void that our universe would be expanding into? After all we don’t know for CERTAIN that it IS a void. Perhaps there is SOMETHING out there and the MASS of that something is “attracting” via gravity our universive towards it (hence explaining the increasing acceleration that scientists calculate our universe is experienceing).

That’s correct. The position of the quasar, or whatever, was 13 billion light-years away from the Earth’s current position in the Universe.

No. Astronomers have used the phenomenon called “redshift” to measure the speed of the most distant objects and have found that those objects are (or, more correctly, were at the time) moving at a significant fraction of c - what that fraction is exactly I don’t remember - but the practical result is that the object that emitted those photons is millions, or even billions, of light-years away from the position it’s being ‘observed’ in now. All we’re seeing is what amounts to a ghost of the object itself.

I think it more than likely even the fastest, most distant objects are losing speed, but on the scale of time needed for significant changes and events to happen with objects of that size, thirty years isn’t even enough for an eyeblink. Our distant descendants - as far in the future from us as we are from, say, the Australopithecines - will have to look and see what’s changed.

The comparisons would be meaningless - parallax (the method you describe) calculates distance, while “redshift” calculates velocity. And again, we’re not looking at the object itself, but the traces it left behind in the form of photons and other energy particles. It’s kind of like reading a broadsheet from 1604 on your morning commute tomorrow. Very, very old news.

It’s entirely possible there is sentient life out there already doing so. It’s only taken us 5 billion years to get to this point; how much further could intelligent life get given three times as long?

Careful there. The Universe is expanding, but it’s not expanding “into something.” A hypothetical observer in this 13-billion-year-old-galaxy would “see” a Universe pretty much the same as we see here, with the Milky Way at very high redshift.

A good analogy is an ant on the surface of a balloon that’s being inflated. The ant sees points on the balloon getting farther apart from each other, but there’s no particular point on the balloon that’s “expanding outwards.” You might object that the whole balloon is expanding outwards, i.e. away from the center of the balloon; but that’s not a direction the ant “knows about” if he’s confined to the surface of the balloon, and the ant could describe the expansion just fine even if he didn’t know this “third dimension” existed.

IIRC, the universe has expanded about 2000 times in size since the temperature cooled enough to let radiation pass freely. The universe was already extremely large before this time , either due to inflation or the fact that it somehow started big as a legacy of an eariler epoc ( Ekpyrotic theory). However, the time of transaprency is the furthest back we can see, the light then has been redshifted all the way into microwaves, and is called the cosmic microwave background. There follwed a period of darkness before the first stars ignited, and probably nothing from this time would be visible.

The universe isn’t expanding into anything, and there is no ‘leading edge’ to look into. The increasing accelerating is due to the fact that sometimes gravity can be repulsive, if the energy generating it has negative pressure. What kind of energy has a negative pressure we don’t know yet, but apparently it fills all of space with a weak feild.

values to calculate distance (how else do we estimate the distance to various celestial objects?). My point was to COMPARE the values obtained from “parralax” with those obtained from Red Shift estimation. Of course a problem with the “parralax” method is that it becomes less useful the further the star is away. Therefore, I think that it is only good for stars WITHIN our galaxy. One way to IMPROVE this method might be to send satellites to opposite parts of our solar system (the further the two known points are apart, and the more precisely the angle can be determined to the third point, the more precise and useful the measurement). Another approach might be to place measurment equipment aboard asteroids or comets with well defined orbits (letting them do the work of creating seperation between the two points).

My point about the “matter” beyond our universal “even horizon” was that it MIGHT explain the fact that astronomers observe the universe to be expanding at an ACCLERATED rate (current hypotheses rely instead upon invisible “dark matter”). If we were expanding towards SOMETHING then the mass of that SOMETHING could be attracting our universe via its gravitational field. Of course I think dark matter would still be necessary to explain observed rotation WITHIN galaxies as well as other observed celestial events.

Ah, you have a slight common misconception here. Dark Energy, not dark matter, is the mysterious ‘stuff’ that supposedly makes the universe expand. Dark matter is normal stuff like the planet Earth or asteroids and other objects that don’t emit light.

As for your question, there are theories that subscribe to matter outside of our universe to explain the expansion, but they haven’t won out over the other theories yet. For instance, one theory describes our universe as a sort of 2 dimensional hologram (3D if you count time I believe) that is situated around other holograms. This thoery says that gravity can travel between holograms and is responsible for the observed expansion.

I’m only a sophomore physics major and am much sleep deprived, so please correct me if I’m wrong.

It didn’t pass us because the object is 13 billion light-years away from us now, as it was when the light was emitted.

It is not that the object is “moving away” from us. It is simply that the amount of reality between us and the object has grown.

Think about it this way. 13 billion years ago the object released some light. Between its emission and our reception of it, any two points along the “light wave” will have been stretched farther apart than they were when emitted, because the expansion of the universe. Hence the light when perceived is at a longer wavelength than when emitted, meaning the perceived frequency will have been shifted in the direction of the red end of the visible light spectrum.

Excuse my ignorance, but isn’t energy and matter convertible? Anyway, I’ve always read the theories of WIMPs and MACHOs beng referred to as “dark matter.” My question (to divert from the OP slightly) is how can this dark energy/matter make the Universe expand? I’ve always read that this “matter” was theorized based upon the possibility of a closed universe. Calculations based upon all the matter/energy we can detect indicate that there is no where near enough to prevent the Universe from expanding forever, but that WIMPs or MACHOs can provide this additional matter/energy. Can you please enlighten me here? Thanks.

Ok, I appreciate the question - since it’s one that I have struggled with for a while, too. Here’s my problem with it: We are told that we are seeing light from the farthest regions of the universe, and that the distance is some 13 billion light years. This is farther than we’ve ever “seen” before. Does this mean that such a star (or whatever it is) was 13 billion light years away from us when it sent that light to us? If so, where is it now? But, if, as they say, it is a picture of the earliest times in the universe’s history, then wouldn’t it be much closer than 13 billion light years? Ah, crap - I can’t even think of the right question. But I want to be in on the conversation. xo

You haven’t provided a quote, link or cite, but I’ll just respond to this one line by cautioning that the word “inflation” has a VERY specific meaning to big bang cosmologists. It refers to idea that there was a period in the early universe when the expansion of the universe was very great and then slowed down, which resolved some discrepancies in the theory a few decades ago (while introducing others). Not all astronomers subscribe to the “inflationary universe” model of the Big Bang, but that does not necessarily mean they don’t think the Universe is expanding.

The technique you are describing is “parallax”, and was conceived as a way to measure the distances to the stars. If you observe a star on some date, and then observe it again 3 to 6 months later, when the Earth is in a different part of its orbit, then the closer stars will have shifted position more noticably than those further away. You can convert this apparent motion into an angle measurement which translates to a distance. It took decades of observations before the first parallax of a star was observed, and it was incredibly tiny.

Distances measured with parallax are stated in parsecs (1 parsec is how close a star would have to be so its displacement during PARallax observation would be as large as 1 SECond (1/3600 of a degree) of arc. It is equal to over three light-years.

Earthbound parallax loses its usefulness at about 100 parsecs. Since the closest galaxies are thousands of parsecs away (and the ones far enough away for expansion to overcome the local effects of gravity are millions of parsecs away) parallax can not be used as a comparison to red-shift.

Even if you had the ability to use parallax to measure out that far, it depends on observations of near and far objects. You couldn’t use it to measure the distance of the farthest objects for this reason.

There is no leading edge of the Universe. We see galaxies “moving away” from us, and any observers there see us “moving away” from them, and nobody is wrong in their perception.

It all gets put together this way. We do our best to determine how the rate of expansion of the Universe, and we have enough reason to believe that this exapnsion rate is uniform except at very distances, where gravity keeps clusters and superclusters together. Therefore, the farther the object, the greater the redshift due to expansion.

We then do as comprehensive a survey of the sky as possible, and figure out which objects have the greatest redshift (are the farthest away). Those are the oldest, because light only goes so fast.

We deduce that since the farthest we can see into the universe is 13 billion years, then the Universe is that old.

There are undoubtedly objects farther out than 13 billion light-years, but becuase the Universe is only 13 billion years old, the light from them has not reached us yet. However, so far, at scales on which these things matter, the Universe has turned out to be more or less uniform in the nature and distribution of its visible matter, so we expect no surprises as these farther objects come into view.

We could be wrong about that last item, but there is currently no way for us to confirm it by observation. It could be that some luminous stuff 17 billion light years out is completely different than the rest of the Universe, and affects the behavior of the visible Universe in such a way that explains away all the wierd discrepancies we see that cuase us to come up with string theory and dark matter, etc. It might even be possible that someone could develop a theory along these lines.

But we’d have to wait another 4 billion years to find out for sure if they were right.

Ok, let me be a little clearer. Dark matter is nothign weird or strange or etxraordinary. You are dark matter. Most matter you come into contact with every day is dark matter. A star is not dark matter. The difference is that the star is undergoing nuclear reactions and giving off energy. However ,it is still made of the exact same stuff as dark matter. We call stuff dark matter because we can’t visibly observe it (at least not easily) from far away.

Dark energy is a sort of ‘negatvie energy’ which would therefore have to come from matter with negative mass. Don’t ask me how. This is not the same as anti-matter, also a common misconception.

Hence, if you convert dark matter to energy, you get normal energy, not dark energy. At any rate, the reason tehre’s all this tlak about ‘dark matter’ is because scientists began looking at the behavior of galaxies and other such things and said 'this behavior doesn’t agree with our calculations oft he masses of these systems from the matter we can see, so it must be the extra dark matter that is causing the discrepency." As it turns out they’ve made some mathematical guesses at how much dark matter is out there based on observations and these calculations gives results that are much smaller than what they should be. Why? No one knows! Again, IANAP yet, so feel free to shoot down what I say if you want.

Dark matter can also be observed by it’s effect on light from even more distant galaxies. If two gravitaional clusters are aligned from our line of sight, the dark matter in the nearer one will distort the light like a lens. This distortion can be measured to map the distribution of the dark matter. The mapped distribution exactly matches what the theory said it should. Most dark matter cannot be explained by ‘ordinary matter’ though, it is something we simply cannot detect at this time. There simply aren’t enough dust clouds and small planets to explain most of the missing mass.

Dark energy on the other hand is really unknown. It has negative ‘pressure’, not negative energy. This means that the gravitational force arising from it’s mass is negative, and repulsive. ( I prefer to think of it as positive, and normal gravity as negative, but that’s just me). The supposed cosmological constant as calculated from theory is 10x10¹²º times what it actually appears to be. That’s a rather large error, so my view is that the dark energy cannot be a cosmological constant and must be something else entirely.

We think lots of dark matter exists because it’s difficult to explain the formation and shape of structures all the way from galaxy scale up to supercluster scale without it. It’s been estimated that only about 0.5% of the universe is luminous, and hence, visible. The other 99.5% must be something, and one component of that something is dark matter.

Actually, there are two kinds of “dark matter”.

“Hot dark matter” might be “normal” matter, but it moves very fast. Neutrinos are the best candidate, and clearly they have (a very small) mass and go like hell. Thing is, the universe can’t be made of too much of it, or small scale structures would be overwhelmed by its motion and gravitational influence. Fortunately, observations support this: Neutrinos only make up a tiny fraction of the total dark matter.

“Cold dark matter” moves slowly, because it’s heavy. It includes “normal” or baryonic matter like the stuff we’re made of (protons, neutrons, that kind of stuff). It also includes black holes, which are pretty exotic, but thought to been formed out of baryonic matter that collapsed under its own gravity. The Big Bang theory has been worked out pretty well up to the tiniest fractions of a second after creation. Using this theory, physicists have been able to predict the prevalence of certain light isotopes (hydrogen, helium, deuterium, lithium), and their predictions match observations to almost absurd accuracy. So we’re pretty confident in the model, up to a point. Thing is, given the rate of nucleosynthesis, if the total mass of the universe were more than about 5% baryonic matter, the ratios of the light elements would be different.

So, 0.5% of baryonic matter is visible. The other 4.5% is clouds of gas and other dark objects like black holes, rocks, dust, brown dwarfs, the so-called “compact halo objects” that float around galaxies. This stuff is hard to see directly, but it’s a safe bet it’s out there.

OK, great, what about the other 95% of the universe?

Nobody knows.

Currently, theory predicts about 70% of the universes matter/energy is “dark energy”, which is currently starting to overwhelm the gravitational attraction of all the “matter” in the universe, causing stuff to move apart at an accelerating rate.

So the other 25% is “matter”, but we have no idea what kind. This “non-baryonic matter” could be practically anything, but there’s strong suspicion that its comprised of extremely heavy particles that only interact with other matter via the weak nuclear force and gravity. The preponderance of this matter is thought to be the “lightest supersymmetric partner”. What the hell is that, you may ask. Well, right now particle physics is in something of a fix. Most of the theory works pretty well, but some things beg explanation, and, without going on a huge tangent, one possible extension of the current Standard Model of Particle Physics posits the existence of counterpart particles for every particle we know. This theory is called “Supersymmetry” or SUSY for short, and gives us a whole new zoo of subatomic particles to look for. The SUSY partner of the neutrino is the neutralino, the partner of the photon is the photino, and so on. The theory works so well, physicists are reluctant to let go of it. The only problem is, no one has ever observed any of these SUSY partners. So, figure the physicists, they must be incredibly heavy, so that we can’t create them in our most powerful particle accelerators. However, even as heavy as they are, there must be a lightest supersymmetric partner that all the others would decay into. Maybe it’s the photino, or the neutralino.

Whatever it is, there’s more of it than any other kind of matter, apparently, and it seems to clump around galaxies. If gravity works the way we think it does, its concentrations can actually be mapped using the visible structures we see. Our galaxy might sit in a spherical cloud of neutralinos, and trillions upon trillions of them are passing through you right now, without you even knowing, so little do they influence normal matter. Since we’re orbiting the galactic center in a particular direction, and it’s assumed these particles just drift around randomly, we ought to be able to detect them moving through us (and everything else) from our direction of motion. If just one of these particles would interact with a normal atom and impart some influence on it, it might be possible to detect it. People are trying to do just that, but so far without much luck.

Maybe it’s all just a bunch of baloney. As weird as it all sounds, however, the alternatives are even weirder. No one can deny that what we can see or account for in theory as “baryonic” matter is pathetically insufficient to account for the motions of, say, the spiral arms of galaxies, or their orbits around one another, or their propensity to cluster into enormous filaments and sheets that stretch across the cosmos like a giant foam of bubbles filled with dark, nearly empty voids. If not dark matter, what? About the only other explanation anyone can come up with is that the laws of gravity as we know them only work at short distances. In truth, we’ve only tested gravity at very short distances. We have no direct measurements of how it behaves on galactic scales. Even weirder, observations of Voyager’s faint signals from the edge of the solar system indicate its in a different place than Newtonian dynamics would predict. No gas leak or other glitch or influence is sufficient to easily explain the discrepancy. So, now we can’t explain Voyager’s trajectory using the firmly established laws of gravity, and all conventional explanations have been ruled out! How can this be? Could dark matter or dark energy be to blame? Do we not fully understand gravity? Do we need a “modified Newtonian dynamics” (MoND) theory?

Whatever the answer, what we DO know, currently, is that our picture of the universe is far from complete.

Fascinating thread, everyone.

I remember the fun I had with my cheap telescope as a kid, and the thrills I felt the first time I actually SAW Jupiter, in her striped beauty… and her twinkling little moons…

I almost became an astronomer that day. Reading this stuff, I’m glad I didn’t. WAY over my head. Interesting, nonetheless…

I don’t think so. I’m not a scientist of any sort, but it appears to me that as the “reality” between the objects has increased, and continues to increase due to expansion of “reality,” the distance between the objects will also increase. The object is now 13 billion light-years away, and in the future it will be 14, 15, 16, etc. billion light years away.

My take on this, and correct me where I’m mistaken (a superfluous remark), but at the time of the “Big Bang” (creation), an enormous amount of energy existed and the Universe was extremely hot. Over time, the energy condensed (and the Universe eventually [a matter of a few seconds, I guess] became “transparent” with the formation of matter. The remnants, however, of that energy permeates the Universe and is now called the “background radiation from the Big Bang,” at a temperature of a few degrees above Absolute Zero. That energy has expanded with the Universe. And in so doing it has dissipated to the low temperature it is now. However, when matter finally condensed, it became embedded into the fabric of space, and every object in the Universe is regressing from every other object at the same rate. The amount of the “reality” has increased between every object, and so has the distance.

As to “dark matter,” it has been years since I’ve kept up with these theories, but the last I heard there were two theories (WIMP: weakly interacting molecular particles, IIRC, and MACHO: massive, something, the rest I’ve forgot), but apparently from these posts, the WIMP theory has been forsaken for the MACHO theory. Such is life. Wimps always lose out to the machos.