This is probably a very basic question for cosmologists, but puzzles me.
Hubble has taken pictures of the oldest and most distant galaxies ever. This is very faint light from the earliest objects in the universe ever seen.
My question is; if this is the light just reaching us from these far and earliest objects, how did we get here before the light did?
Is this a case of the light “wrapping around” the universe a number of times and passing us for the millionth time? If so, how then can we meaningfully call them ‘most distant’? Without knowing the size of the universe surely we can have no idea of the distance the light has travelled or when it started its journey?
I’m sure that somebody will be along to explain it better, but as I understand it, it’s because space itself is expanding. So even without moving through space, everything is still getting further away from everything else. The light is travelling towards us at c, but as it travels, it has more and more distance to get through.
Going back to the “dots on a balloon” analogy, imagine several dots painted on a balloon. As the balloon is blown up, the dots get further apart, even though they are not moving through “space” (the fabric of the balloon). Now imagine an ant trying to walk from one dot to another. As the balloon gets bigger, the ant has much further to go, so its journey will take longer.
I think this analogy still holds, even when you are talking about light speed, but I may be off base. Anyone?
We are at a point x. These galaxies are at a point y. The distance xy is huge. Light, the only way we have of knowing that these galaxies exist, travels at finite speed. Hence, it will take some time for the light from point y to reach point x, in this case a few hundred million years. Thus the effect of looking back in time as we go to deeper and deeper images of our universe.
There’s no question of us getting here before the light did. Consider the surface of a balloon as an analogy for the universe. The galaxies are dots on the balloon. As the universe expands, galaxy x and galaxy y move further away from each other. Light travels on the surface of the balloon. If a light ray from y sets off, its going to take some time to get to x because of the expansion and the distance, yes? So, when the light ray reaches us, we see what galaxy y looked like z many millions of years ago.
In the moments after the big bang, the space which contains the universe inflated at a rate greater than the speed of light. By the time the universe was 200,000 years old, it was about 26 billion light years in diameter. The expansion rate has slowed since those heady early days and is now on the order of 80 kilometers per second per megaparsec (Hubble constant).
I’m not sure of this, but I think the light has always been here. As the universe expanded it cooled and at some point while it was still quite “small” the earliest discrete objects, quasars as far as we know now, formed and started emitting radiation including light. Our assumption is that the speed of light was the same then as it is now. Our estimates of the age of the universe is based on that. So it wouldn’t take long for the quasar light to fill the universe at that time. As the universe expanded so did the light from the quasars and at some point things like our galaxy, solar system and the earth condensed out and it was surrounded by the quasar light.
Our measurement of the red shift of the light from any celestial object is a measure of how much the light, and therefore the universe, has expanded since that object was formed and therefore its age.
Now that r_k and I have gotten the ball rolling maybe someone who really is sure of the answere, if anyone is, will join in and make the necessary corrections.
Er, no. If that was the case, then at every frequency we would see a hodge-podge of sources. Which we don’t.
Quasar light never filled the universe. See my explanation above. The speed of light is finite, and because it is finite, it takes time for light emitted from point A to get to point B. Hence why we only see some galaxies now, and that it takes very long exposures to see the oldest light - its faint.
Our estimate of the age of the universe is based on the expansion of the local universe, where we can asssume that the speed of light is constant. Essentially, we measure how fast an object is receeding from us, and plot it against the distance it is away from us. The gradient of this plot gives us the Hubble constant, the inverse of which gives us the Hubble time, or the age of the universe.
Um. The obvious answer (which, of course, might be wrong) is that this light was emitted more recently than that; it’s light from when these objects had already traveled billions of light years. The light from the very beginning went past our region long ago, before we were here, and light kept passing us before we had the technology to detect it. One might say, if one were not all caught up in relativity, that the object doesn’t look that way any more, that it’s even farther away now than it was when it emitted the light we’re seeing now. One might also point out that the expanding universe causes objects to recede from each other, causing a wavelength shift to the red, and that much of the astrophysics research of the past century has been focused on finding the exact relationship between distance and red-shift.
You’re wrong. The light was emitted a few hundred million years ago. I’ve said this twice in this thread already, and I’ll say it again - light travels at a finite speed. It takes time for light to get here. If the light was emitted more recently, we’d know about it in the spectrum of the object.
We know the relationship - its derived from general relativity.
Slight correction. Yes, there has been research into getting the exact relationship, but that’s been done mainly for accuracy. Now that we know Hubble’s constant to within 7%, as well as various cosmological constants, we can actually determine cosmological distances rather well.
No…kidding, Sherlock. The OP implied that light was traveling for a LONGER TIME than the distance would require – billions of years. A few hundred million years IS “more recently.”
I said the EXACT relationship–the value of the Hubble constant, in other words, which can only be determined experimentally.
Yeah - sorry. See my post above.
The thing is, we’re seeing these galaxies just as they started to emit light - they’re in their initial star forming periods. We know this because the redshift corrected spectra are actually in the UV band, which is very sensitive to star formation. In fact, the measure of UV a galaxy gives out is used as an indicator of star formation. So, the notion that the light from the “very beginning” seems strange to me. This is light from the beginning.
Sorry if I sounded snarky. We’ve just been discussing this in the office this morning, and sometimes its hard to forget that not everyone has the same inside information.
So, to summarize and cut to the point; we got here before the light due to the expansion of the universe averaging a speed greater than the speed of light?
So my wag about the light travelling around the universe and us catching it on a ‘circuit’ of its journey was totally wrong. And obviously if we are expanding faster than light, no light will ever ‘wrap around’ the universe. (Unless the universe slows and contracts.)
My next question is therefore probably a bit trickier: if the universe is expanding, why didn’t/doesn’t this light get swept along at the same time? Why did the particles that make me and Hubble get here due to universal expansion, but the particles that make up the light didn’t?
OK, back to the surface of a balloon analogy. When the balloon is at some small size, a galaxy at x emits two wavecrests of light - one at time t and the second at time t+a bit.
The universe carries on expanding during this, and the light sets off on its merry way. Because its travelling along the surface of the balloon, the wavecrest stretches with the expansion of the balloon, but still carries on travelling towards us. So, yeah, the light does get stretched with the expansion of the universe as it travels.
But by the time the light has travelled the distance from x to us when the balloon was small, the balloon has expanded some more, the light needs to travel a bit further to get to us. Which isn’t too much of a problem as we’re expanding at a speed much slower than the speed of light.
Its kind of like this. Think of us as fixed blobs on the surface of a balloon. Think of the light ray as someone with a pen who starts drawing on the surface at a certain time whilst the balloon is still expanding. As the balloon expands its going to take longer for that line being drawn to reach us.
I don’t think I understand your point. If I have a light bulb in a room I can see light from the bulb no matter where I am in the room. In that sense the light from the bulb fills the room. If another light bulb is added I can also see it from anywhere in the room. Its light also fills the room but I still see two separate and discrete points of light. And, of course at various frequencies from microwaves to gamma rays we do see all sorts of sources everywhere we look in the cosmos. Would that be a “hodge podge”
I mentioned that someone who is sure of the answer might come along. You seem sure enough but I’m not sure if you have the answer since your point is not clear to me.
The universe isn’t expanding at the speed of light now and after the initial rapid exansive phase, assuming such a thing existed, hasn’t done so. It seems to me then that the light from the early sources like quasars has to be everywhere in the universe.
Answer: because the light from the bulb has had time to reach your eyes. Light travels at 3.0x10[sup]8[/sup]m/s. Now, for it to fill a room, that will take no time at all. For it to fill the visible universe, even when its at a fraction of its age? Going to take some time.
Yeah, but we see discrete sources, emitting at various frequencies. By your argument - “the light should fill the universe”, we should just see flat, featureless emission surely, with no point sources. The only place we see that is in the microwave waveband - with the cosmic microwave background. Radiation emitted when the universe was very very young, pre-inflation, pre-stars, pre-galaxies, pre-anything complex, when the universe was small enough for radiation to fill the universe in the same way that the light from a lightbulb fills a room practically instantaneously.
Why? I don’t understand why you think this. The distances involved are just too huge for light from distant old objects to “fill” our universe. If that was the case, the night sky would be bright.
Sorry I don’t fulfil you’re critera for someone who “knows”.
I think I am understanding what is being said but am not clear on (at least) this point:
According to the media, these objects appear to be about 13 billion light years away and represent their state at the beginning of the our universe. But are we actually seeing a record of what happened 13 billion years ago or a record that occurred some time sooner? The continuing expanding balloon example would seem to indicate some time less than 13 billion. If so, then how do we know the age of the universe?
Perhaps I might mischeviously throw a spanner in the works of the simple explanation, and possibly answer the original question as well.
The light from these galaxies has been travelling for perhaps 13.5 billion years. This is called the Light Travel Time Distance.
However the universe was much smaller (or rather less expanded) when they actually emitted the light; these galaxies were only about 2 billion light years away then. This is still apparent in their angular diameter, or so I am led to believe, so this is caled the Angular Diameter Distance.
While the light has been moving toward us, the galaxies themselves have been moving because of the expansion of the universe in the opposite direction.
They are now actually a very long way away, more than 30 billion light years, and no matter how hard we tried, we could never get to them (apparently).
This one is called the Comoving Distance.
Thanks! That link helps a lot. So we are actually seeing the universe as it was 2-3 billion years ago, even though these far light sources are estimated to be 13 billion light years away, right? Will we ever see more of the universe? Does the circle of visibility increase as time marches on?
Er, no - we’re seeing the universe as it was 13 billion years ago - when the light was emitted. Provided your definition of billion is a hundred million (which as I’m assuming you’re American, it is), I got terribly confused there for a moment.
As time goes on, we will see more of the universe, as the light from more and more distant objects reaches us.