The Universe Beyond Our Reach

Can you explain something: inflation driven by "dark energy" has only begun relatively recently, right? So the cosmological constant has presumably changed over the lifetime of the universe? And it definitely changed for the inflationary period. So why would we expect that the current accelerating expansion is the last word, and there could be no further changes to the cosmological constant?
Here's something else. The age of the universe so far is a tiny, insignificant fraction of the duration we expect the universe to be able to support sun-like stars where planets can form and life like ours can develop, right? And for most of the period where life like ours is possible, galaxies beyond our local group will be unknowable, according to this post. So it seems incredibly unlikely that we should find ourselves in the tiny sliver of time where we can see the rest of the universe. Yet here we are! Do any astronomers use this line of thought to argue that something is likely to be wrong with the assumptions that lead us here, and that we should try to find an interpretation of the universe where we aren't so incredibly privileged in time? (You can't use an anthropic argument against this, and say "well if it wasn't like that we wouldn't be here"--because we very well *could* be here in an island galaxy, we don't need any remote galaxies for life to develop and look out at the universe.)

@uncleMonty (the second part only): I'm not sure that characterising "100 billion years" as a "tiny sliver of time" (compared with the mere 13.7 Gy which have passed so far) is really fair :-)

On a more serious note, our presence here and now was the subject of a comment in one of Ethan's posts a couple of weeks ago.

If we assume (and it is an assumption!) that abiogenesis is straightforward chemistry, likely to happen anywhere that the right conditions are met, then our appearance here and now should not be a surprise. Rather, it is a simple consequence of the conditions only being right (e.g., gas and dust clouds with sufficient metallicity of C, N, O, P) in the last few billion years, after a few cycles of star formation and nucleosynthesis.

A good test of this hypothesis is whether the metallicity of galaxies varies with redshift: I'd expect to see vaguely solar or Milky-Way values out to z~1, but not much older than that. I'm not an astrophysicist, so I'm not sufficiently versed in the literature to know whether I'm right or wrong.

The other half of this part of your question is "why did dark energy 'turn on' just now, rather than at some other time"? As best I understand it (and this is precisely Ethan's specialty, so I'm speaking quite out of turn here :-), this relates to competition between the mass density of the Universe slowing down the expansion, vs. dark energy accelerating it. Earlier in time, when the Universe was "smaller" (yeah, yeah, I know), the mass density was high enough that gravitational slowing dominated over dark-energy acceleration. A few billion years ago, the expansion had gone on long enough that the mass density dropped below threshold, and dark energy could "take over."

It is an interesting coincidence that the two halves of your question occurred at roughly (within a billion years or so) the same time. But I think, because the physics involved are disjoint, that it really is just a coincidence.

I don't think that the idea is that Dark Energy inflation just switched on but that we only "know" it is currently occuring. If it started earlier, then the universe is younger than the 14 billion years currently given it and that puts a limit on how "old" Dark Energy driven inflation could be (you can't have a galaxy younger than its stars).

There was a while ago a post by Ethan about this. Appropriate google-fu on this site should be able to find it amongst the other "inflation universe" threads.

@uncleMonty: If my calculations are correct, the acceleration of the expansion started about 6.5 billion years ago - I don't know if you consider that "recently"... ;-)

And no, the cosmological constant has not changed. What has changed is the mass/energy density of matter - that has gone down. So the decelerating effect of matter became less and less with time, until the accelerating effect of dark energy began to dominate.

To answer (from a cosmologist), the "takeover" of dark energy has begun only recently, but dark energy has always been there and has always had the energy density that it's presently measured to have. (At least, according to our best measurements.)

The dark energy density today, however, is 73% (more-or-less) of the total energy density in the Universe. But the other forms of energy present in the Universe are dependent on the size/scale of the Universe. So if you halved the size of the Universe, the energy density of dark matter and normal matter would go up by a factor of 8, meaning that you don't have to go back very far at all for dark energy to appear negligible.

In fact, because the energy density of photons actually changes more quickly as the Universe expands (as scale factor^4 instead of scale factor^3 for normal matter or scale factor^0 for dark energy), at one point in the distant enough past the Universe was dominated by radiation!

I wrote a post on this a couple of years ago with some very informative illustrations/graphs here: http://scienceblogs.com/startswithabang/2010/12/30/whats-in-the-univers…

The two graphs of interest (that I made myself, so I know they're right) are of relative energy density: http://scienceblogs.com/startswithabang/files/2010/12/rho_vs_a.jpg and of fractional energy density: http://scienceblogs.com/startswithabang/files/2010/12/EthanCreatedEnerg…

Michael Kelsey wrote:
"@uncleMonty (the second part only): I’m not sure that characterising “100 billion years” as a “tiny sliver of time” (compared with the mere 13.7 Gy which have passed so far) is really fair :-)"

I think you must have misread, I believe I got it the right way around--the tiny sliver of time that has passed so far is insignificant compared to the ages to come.

"On a more serious note, our presence here and now was the subject of a comment in one of Ethan’s posts a couple of weeks ago. If we assume (and it is an assumption!) that abiogenesis is straightforward chemistry, likely to happen anywhere that the right conditions are met, then our appearance here and now should not be a surprise."

I didn't mean that it's surprising that it's taken this long--I understand that (though I appreciate your explanation). What I meant was it's a "surprise" that our period in the universe is so near the beginning. Let's say we're in the first thousandth of the universe's age. This is surprising in the same way that if we were 10 thousand people in a stadium, all given different random numbers between 1 and 10,000, it would be a surprise to find ourselves with a single-digit number, wouldn't it? So given that there are many, many more periods of the universe that come later, when other galaxies have receded from view, it seems unlikely that we're right here right now (instead of later, when life could also develop around a newly formed star in the far future). So by the principle that explanations of the universe shouldn't involve us being in a highly privileged position, I would have thought this was an argument that maybe our era isn't so unique and there may be hope for some misunderstood aspect of cosmology to prevent the ultimate recession of non-local galaxies.

Does that make sense?

I really like this blog.

And thanks for the explanations, especially Ethan's, of why it's not the case that the cosmological constant has changed or that dark energy has only "recently" been turned on--if I understand correctly now, it's that only as the universe gets bigger that the relative strength of the expansion due to dark energy increases in importance and is no longer insignificant compared to the tendency to slow down due to gravitation.

@Ethan & Wow: Thank you both for confirming my understanding of the DE vs. matter competition (and for describing it better than I did!). And thanks also to Ethan for the plots and the deceleration scaling laws; that makes it much clearer.

@uncleMonty: I think I understood your "surprise" comment correctly, and I was attempting to point out that it isn't (and shouldn't be) a surprise at all. Your analogy of drawing lots is a good one, but I think you're misinterpreting it. If the distribution is uniform and limited -- you say you will definitely pick one winner from the group of 10,000 -- then it is ABSOLUTELY CERTAIN that someone in that group of 10,000 will be chosen. The odds of any specific person being chosen is tiny (1:10,000), but that's not the right measurement to ask about our existence.

If abiogenesis is common (the assumption I made), then _somebody_ had to be first (or early). If we were not the early one, then we wouldn't be here to argue about it. But that doesn't make us any more or less probable (which is another way of saying that you cannot assign a "probability" to a post facto occurrence).

I've got to say that each post on this subject brings me closer to a place where I might attempt to "intrigue" my math/science interested great niece into making the "harder" sciences one of her life's interests.

Do you, Ethan or any of the many deep thinking commenters here have a good intro physics/cosmology book suitable for a 10th grader? I have a couple (that I already have) I'm thinking of giving her, "Why does e=mc2?" and "How to Teach Physics to Your Dog". Any others? Bear in mind that as a 10th grader she has more interest in members of boy bands than in what she'll be studying when she goes to college.

It is a little more general than hard physics but the "Science of Diskworld" series has a few things in it that will educate and entertain.

"The 4% Universe" has a lot of the history and very little of the maths and also gives props to the women who worked on these problems and got feck all plaudits for it.

Cosmos of course.

And more modernly, with the advantage of an ex pop star Brian Cox running it is the BBC "Wonders of the Solar System" and "Wonders of the Universe" series.

More involved books aren't really something I'd want to try to advise on because it is very particular. I'd once read and understood a little bit of a book on computer processor design when 8. The book went on about how to create the Uncommitted Logic Array of early CPUs and so on. About 12 years beyond my reading level. I found it interesting even if most of it made no sense to me.

You have to want to read those books though. Telling someone to read it will be off to a bad start...

@Michael: okay, I understand what you mean now about the sliver--I meant the duration of being able to see non-local galaxies, compared to the age of the universe when the last solar system forms, is just a sliver.

About the lots analogy: sure, someone has to win, and I won't be surprised if *you* win (or some other non-me person)--but I can be surprised if *I* win, as that was a very unlikely occurence compared to me not winning. The analogy is that we (humanity) should be surprised that we find ourselves at the very tip of the universe, so to speak. I don't doubt that the Friedmann equations are correct or that we're at the tip, but doesn't it cry out for explanation? I don't buy this line of arguing: "If we were not the early one, then we wouldn’t be here to argue about it."--sure we would, we'd just be here in the year 572 Billion instead of the year 13.6 Billion.
I don't believe I've found a flaw in cosmologists' reasoning, I just haven't understood yet why this doesn't constitute a paradox (like an interpretation of the big bang that sees all other galaxies rushing away from us in a special position at the centre--reality is that we don't have a privileged place.) Thanks for your replies!

Reading here about the increasing expansion rate and "100 billion years in the future"... made me think of something: Has the Big Rip idea been discredited? I remember it said that in 20-30 billion years, the acceleration of expansion would pull apart the galaxy, then separate planets from their stars, and finally, destroy atoms.
Even if the Big Rip is after 100 billion years, is it still throught to eventually be the ultimate fate of the universe where space expansion overcomes all forces, finally tearing atoms and nuclei?

I get your argument that getting a one digit number would be rare, but you could also get the # 9,999 and say that that is even more rare. The truth is the number 1 is just a rare as numbers 2 3 and 6 and 9,999 or 10,000 as each one holds only 1 unique value. Being alive at this spacetime coordinate (13.6b yrs:Earth) is just as special as being alive at another (999.999999b yrs.:Gleise 581g).

"The analogy is that we (humanity) should be surprised that we find ourselves at the very tip of the universe, so to speak."

Monty, you're defining "tip" as "the current age of the universe".

But most of life on this planet and most of humanity have not lived today. And in 100 years none of us here reading this codswallop will be here, but this will be a new "tip" that some credulous fool considers to be so special.

Please stop trying to make a religion out of facts.

We are alive today.

This is no more special than finding that out of all the number of legs you COULD have, you have exactly two, therefore two must be a mystically significant number.

Ethan, perhaps you could help me understand something? I get that dark energy is the energy of empty space, but why does it have a repulsive gravitational effect rather than an attractive one? As far as I can see the only difference between dark energy and ordinary energy/matter is that dark energy is uniformly distributed through space and its energy density is constant, while matter/ordinary energy can clump and decreases in energy density as space expands? How does that lead to repulsion?

Wow wrote:

> Monty, you’re defining “tip” as “the current age of
> the universe”.

Not really. If the era in which stars are still around to support life on planets is to last ~10^14 years, then we live roughly in the first 0.1% of the duration of the universe. (And we will remain in that first one thousandth of the universe's duration for much more than the hundred years you mention.) (By the way we shouldn't count the first 4 billion years or so, which is understood to be likely as the earliest that intelligent civilizations could develop. So we're in the first 9 bya out of an expected 10,000 bya.)

> But most of life on this planet and most of humanity
> have not lived today. And in 100 years none of us
> here reading this codswallop will be here, but this will
> be a new “tip” that some credulous fool considers to
> be so special.

Credulous fool? Nice. But look: 100 years ago we had no idea of the age of the universe, right? Imagine you knew then that it would have a finite duration, but you had no idea what that duration would be--that's the situation humanity was actually in, at least if you believed in a finite duration for the universe. Statistically speaking (i.e. without any cosmological knowledge) you could estimate the likely duration of the universe given its current age (assuming you know that)--you can estimate the chances anything will still be around in x number of years given its current age. From that point of view, it would be extremely surprising to learn how much longer the universe would last. I mean can you honestly say that you would not be surprised to learn that we live in the first 0.1% of the universe's projected total age in which planetary life is possible? Do you really think that finding that out would be no more interesting than finding out we were currently at 61.3% of the universe's age? Sure, it's just as likely that we're in the first 0.1% as that we're in the 61.3rd %, +/- 0.05%. But one of those is a much more interesting fact than the other. And of all the civilizations that ever develop far enough to figure out the age and expected duration of the universe, only a tiny fraction of them will be similarly privileged.

> Please stop trying to make a religion out of facts.

That's a total mischaracterisation of my question. Apparently it's a more sophisticated philosophical question than you understand. Go look at the wikipedia article for "doomsday argument" which describes a similarly thorny problem.

> We are alive today.
> This is no more special than finding that out of all
> the number of legs you COULD have, you have
> exactly two, therefore two must be a mystically
> significant number.

Nonsense.

"If the era in which stars are still around to support life on planets is to last ~10^14 years, then we live roughly in the first 0.1% of the duration of the universe"

Whiskey
Tango
Foxtrot????

Go away you loon.

And it just goes to show that there is no small corner of the internet, no blog thread on such an interesting topic, no commentariat you would so expect to be intelligent, that you won't very soon find some moronic ignoramus who resorts to insults as soon as he (for it is invariably a he) is unable to participate articulately and on topic.

I point out that there is nothing even potentially loony about the sentence you quoted--all of what I wrote there is in agreement with the current state of knowledge. For the figure of 10^14 years before stellar formation stops see "A dying universe: the long-term fate and evolution of astrophysical objects", Fred C. Adams and Gregory Laughlin, Reviews of Modern Physics 69, #2 (April 1997), pp. 337–372.

From there I imagine you can do the maths yourself, since you were so eager to tell everyone how far past the expected reading level for your age you are.

Tell me, are you always astounded that no matter how much water the pond contains, it ALWAYS fills exactly the same shape as the banks?

Here's an observation that'll blow your socks off.

No matter how long your life is, you always are born *at the beginning of your life*.

Isn't that amazing?!?!?

"Tell me, are you always astounded that no matter how much water the pond contains, it ALWAYS fills exactly the same shape as the banks?"

No, for if it were otherwise it would be breaking basic laws of physics.

"No matter how long your life is, you always are born *at the beginning of your life*. Isn’t that amazing?!?!?"

No, for that is the very definition of the beginning of one's life. These are infantile questions.

Here's a thought experiment for you. I'm going to put you to sleep for up to ten thousand days (almost 3 decades). I'm going to wake you once every morning, and after a few minutes I'm going to put you back under with a drug that makes you forget you were ever woken. EXCEPT, there is a randomly chosen day from among the ten thousand, at which point I will tell you the experiment's over and you can go on with your life.

Now, the experiment is underway. You wake up and say hopefully "Is this the day??" wondering how many times you've gone through this already. I say "YES! The experiment is now over". You say "How many days did it last" and I say "only one! The randomly selected day was the very first one!"

I (still the experimenter) seem honest, and I can't quite prove to you that it was only one day (e.g. because we're in a remote location with no external communication), but I wave my hands about the positions of the planets, and point out you have no stubble, your reflection in the mirror does not really look older...

But come on! Maybe this was all an elaborate hoax, there was never really any such drug, and I just wanted to see how you reacted. Or maybe I was bored and fraudulently ended the experiment deliberately. Or maybe at the last minute the research ethics board told me I couldn't continue. Or maybe I fooled you by shaving your stubble, and pulling the wool over your eyes about the planets and it was actually day 4,902. Because really, what were the odds that the experiment would end on the very first day? (Obviously if I'm being truthful they were 1 in 10,000.) It was no more or less likely than any other day, but still--that doesn't feel satisfying.

I imagine you can see the analogy. A (somewhat) sentient being like you, posing the question of their place in the timeline of the universe, could come into existence at any point during the 10^14 years we expect the universe to last. (Obviously, it wouldn't be you if you were born on Gliese d in the year 572 billion.) Your scientists find out it's actually *really* early in the universe. Well, you don't really doubt them, because it all seems to hold water, and you're even something of a scientist yourself, so you understand how they've done it--but maybe you still wonder if there's not some assumption that we all got wrong that accounts for this very unlikely finding. After all, you've been trained to mistrust explanations that grant *you* a privileged position in the order of things.

Returning now to the present. Having said all that, *I* don't believe the science is wrong, not about the age of the universe. But I wouldn't be surprised if there were scientists (even non-crackpot ones) who still sought alternative accounts that don't require we be special (and one in a thousand *is* special, whatever you say).

"No, for if it were otherwise it would be breaking basic laws of physics."

But isn't THAT amazing??? In all the ways things can go, not breaking the laws of physics is how they go!

Genuinely, you're looking at the fact that 4.5 billion years after the earth formed you're sitting at a computer ACTUALLY ALIVE as being some sort of mystical fantastic "coincidence" that you just MUST tell the world about.

PS what was your idiotic test supposed to say?

It was TL;DR.

Your line

"Not even if we had a relativistic spaceship, not even if we could travel at the speed of light ourselves could we reach it. Our first radio broadcast will never be received by those galaxies, and in fact nothing we do from now on can ever affect them. They’re already gone."

That line really cuts to the heart of it. The universe we perceive is not only bound to change--it's not even there. This theme is often repeated in science and philosophy. Nietzsche railed again and again against philosophers, theologians, and scientists who didn't even realize their obsolescence leading to his most famous line--"God is dead, and we have killed Him". It's the same as economists and bankers in the 1920s who didn't realize that the Haber-Bosch process and fractional reserve banking had fundamentally changed global capitalism. It's strange to consider that the universe we see isn't even there. It makes me wonder how many of my other beliefs and perceptions are no longer worth believing.

Okay, you're right, that was far too long for a pathetic dipshit like you Wow--I've just noticed how you're polluting the other comment threads on this blog, and if you still don't understand how a thought experiment can illuminate, by analogy, you're just too stupid for me to engage any further. Dunning-Kruger indeed--I'm afraid you think you're the smartest one here but you are sorely mistaken, as you lack the ability to read and understand information. That's all for me, I'm done engaging in a pissing contest with a skunk.

"and if you still don’t understand how a thought experiment can illuminate"

You need to think coherently before you can make a thought experiment.

"Thought experiment" != "Making up any old shit and putting 'if' at the front of it"

Your incoherence and "amazement" that we are alive early on when EXACTLY THE SAME can be said if we'd not appeared for another 10 trillion years isn't my fault.

And your thoughtless experiment seemed to want to have time travelling backward.

If someone wasn't alive 10^13 years after the big bang, they can hardly be conned into thinking they were alive 10^10 years after the big bang.

And it just goes to show that there is no small corner of the internet, no blog thread on such an interesting topic, no commentariat you would so expect to be intelligent, that you won’t very soon find some moronic ignoramus who resorts to insults as soon as he (for it is invariably a he) is unable to participate articulately and on topic.

Amirite?

@UncleMonty:
Interesting question.

The estimate of 10^14 years for the lifetime of stars only applies to the very lowest mass red dwarfs, but even without that, we could easily get a lifetime of 10^12 years for a starry universe. So, yes, it does seem that we have arrived "remarkably early".

But I'm not sure this can be used to derive anything further like you suggest. Its difficult to extrapolate from a sample of one.

For example: If you toss a coin 10 times and get 10 heads, can you conclude its an unfair coin ? Not really, as it turns out. You need to go through a large sample of people who were tossing coins and see how often 10 heads are showing up.

In this case, you would actually need the distribution of the appearance of intelligent life-forms over space and time, to see if our occurrence is really unusual or not.

"So, yes, it does seem that we have arrived “remarkably early”."

If we had'nt arrived when we had, we wouldn't be here to say this.

This is no more remarkable than drawing an eight of spades from the pack of shuffled cards and proclaiming that the chance of this is 52 to 1 against, therefore remarkable.

Ethan, thanks for this post and particularly the two jpegs you linked to in your comment.

Until this clarified things for me, I was having trouble shaking the nonsense belief than the dark matter was down to astrophagic civilisations 'hiding food' in the way a dog might bury a bone.

'Wow' reminds me of a me from an alternate reality that stuck at the astrophysics and consequently missed out on 'a few other things'.

There are plenty of robust thought experiments. The 'I can predict which planes will fall out of the sky' experiment is one that comes to mind :)

Jules, any point to that bollocks about you from an alternate reality?

'cos your head seems to be eternally in one?

" In 100 billion years, I should clarify, this will be all that’s left. The local group. Ourselves, Andromeda, and our mutual satellite galaxies. That’s it."

Ethan does like the 100 Billion year plus predictions. I mean why stick your neck out with a mere 10 or 100 year scientific prediction that might actually be tested.

For example if you believe in such and such a model universe that predicts in 100 billion years such and such; BUT ALSO predicts that in 10 years the James Webb Space Telescope will also be able to see thus and thus. Well then you have something scientific to discuss. Will; such and such a 10 year observable thing be observed NOT; will my particular favorite model universe be thus confirmed or NOT?

And of course a 100 Billion year prediction in a universe that is only 13.7 Billion years old has to have some pretty wide error bars. Who needs error bars anyway.

But what exactly is the classification of possible universes that are being seriously thought about by scientists?

Is there a range of reasoned views on the kind of universe we live in?

Consider the following paper:
Time before Time (Classifications of universes in contemporary cosmology, and how to avoid the antinomy of the beginning and eternity of the world) by Rüdiger Vaas, 2004 http://arxiv.org/pdf/physics/0408111v1.pdf

It's a readable paper, but just skip ahead to page 7 figure 1 where you will see Spacetime Diagrams of Different Cosmological Models.

"Unfortunately, if future-eternal inflation is true, all "hypotheses about the ultimate beginning of the universe would become totally divorced from any observable consequences. Since our own pocket universe would be equally likely to lie anywhere on the infinite tree of universes produced by eternal inflation, we would expect to find ourselves arbitrarily far from the beginning. The infinite inflating network would presumably approach some kind of steady state, losing all memory of how it started […] thus, there would be no way of relating the properties of the ultimate origin to anything that we might observe in today’s universe“ (Guth 2001, p. 78). " Hmm, Guth does say some surprising things.

Oh well, in my view, we got the scientific instruments coming on line in the next decade to really understand in a lot more detail the 1st billion years of our universe and our current understanding of our universe today(e.g. dark matter, extradimensions, eternal inflation, particles beyond the standard model). Predictions about the things that we can focus our scientific instruments on in the next 10 or even 100 years will move science forward.

100 billion year predictions on the other hand kill scientific imagination and scientific discussion; because they assume we already know the answer, i.e. no need to test such an obvious prediction.

In the context in which I used it, an alternate reality is a euphemism for simply having made different choices.

My head is real, so any alternative realities my head occupies must also be real, so that would be reality then.

You see, you mock the people who hear voices as if they are 'dumb' for you assuming that they must think it comes from aliens, mind control, god, or psychedelics.

If you were stronger, you'd know where the voices come from.

FORGODSSAKE, let me bring some estrogen here (though I am probably above average in testosterone): Uncle Monty says hey isn't it kinda cool we are so early in the universe. We're here and having conscious awareness of all this in the first 0.1% of the time in which we could have existed in a somewhat similar life form (as far as we know now). I did not think about it that way before and I find it interesting. It doesn't necessarily mean anything but it might. He's just asking, hey could this mean anything? You can say "no I don't think so" without attacking him.

And yada yada about probability. I get it. Learned basic probability in 6th grade (non-US education) plus I was a math major. You're making assumptions about the randomness of the distribution. Maybe it is, maybe it isn't.

So wasting time.

What's cool about being around 14 billion years after the universe starts?

Really.

Bring all the estrogen all you want, but explain why the hell someone would come along and say that?

Every single event that has happened has a 100% probability of having happened.

Big whoop.

We're not making any assumption about the distribution. It is making something up about it to go "hey, isn't it weird that we were born just now???".

So flipping what?

"an alternate reality is a euphemism for simply having made different choices."

So call it that.

We have completely adequate words for that and they don't carry any woo baggage.

@uncleMonty : your reasoning is quite parallel to the question whether we are living inside a simulation or not. Suppose in the far future it will be possible to simulate 'a universe'. Entities living in this universe evolve, have laws of nature etc. Now, there will be thousands of those simulations compared to one single 'real' universe. So, if you are a sentient being in a universe, the chances that you are living in a simulation are bigger than the chance that you are living in the one real universe. You can add to this then the argument that, if we would live in a simulation, it would be possible to see this, as in a simulation you would want to have a smallest uncertanty defined as you are working with bits, and a you would define a maximum speed to limit memory resources etc. Makes it fun to discuss.

Here's a question that's been bothering me since I read this column. Imagine a line between here and Ultima Centauri (some star at the other end of the universe). Along that line--or within a cylinder drawn around the line--there would be a given number of matter/antimatter pairs created and destroyed every second. As that line gets longer as the universe expands, do the number of matter/antimatter pairs increase or remain the same? I'm visualizing space getting stretched out and attenuated.

Now, people, especially woomancers, THIS is a good question.

I don't know 'cos I'm pretty rusty, but I believe the answer is "yes, there will be 'more' virtual particles" thought since they don't exist long enough to be noticed, counting is an odd question to ask.

However, proving that answer means I'd need to dig into the difference between the metric of spacetime increasing and the volume over which a probability is integrated.

I wanted to explain the average amount of matter to people who do not think of such things.

Very roughly because I am using two different models to estimate the average density of the universe, I came out with about 1 grain of sand per 10,000 miles in all directions. If the all the matter of the universe was rearranged into grains of sand and equally spaced.

Is that an acceptable estimate?

Well there are about 10^80 atoms in the visible universe and it's 14 billion light years across roughly.

10^50 atoms per light year cubed.
100 atoms per cubic meter.
to manage 1 milligram of material you'd have to scoop up 10^21 atoms which would take about a cube 2000 km on a side.

John Dumas and Wow,

Sounds like your estimates of the overall matter density of the universe are pretty well in line. 2000 km is equal to 1240 miles. A cube with a side of 10000 miles has 521x the volume of a cube with a side of 1240 miles, so as long as John Dumas' grain of sand has a weight of about half a gram, which doesn't seem too unreasonable, the estimates are pretty close.

OK, this has bugged me, the article says that even if we could travel the speed of light we could never get to certain places. Is that because the "closing speed" of our blue marble and, say, the Virgo cluster, is greater than the speed of light? (Neither object is traveling faster than "c", but if they are each going at 75% then we get 150%, right?)

Thanks :)

JD, it's a little like a real-life Xeno's Paradox.

By the time we travel to a galaxy 10 billion light years away from us, it will have moved so far away that it would now be retreating faster than light.

The distance between things can expand faster than light because nothing is actually moving. Just distances are expanding.

That pic of the local group shows the Fornax, Ursa Major, and other dwarf galaxies as spirals when they are elliptical dwarves.

The age of universe is only 13 billion years. How can one know anything beyond 13 billions light years away. (Please see the paragraph below picture 3). There may be universe beyond 13 billion years, but the light has not reached us yet.

@Len #47: You've got it mostly right! We cannot directly observe any object _older_ than 13 billion years (technically, 13.67 billion years as our best estimate).

That age does not correspond directly to a distance in light years because (a) the Universe is expanding, and (b) the rate of expansion has changed over cosmological time. You can ask a question like "how far away *today* is a galaxy whose light we see is 13 billion years old", and the answer turns out to be about 46 billion light years (because the galaxy has been receding from us during those 13 billion years).

However, I think you're saying something different. How can we know anything about the part of the Universe that is beyond that 13 billion year (or present-day 46 billion light year) horizon? The answer is _indirectly_. The cosmic microwave background is something we measure over the whole sky. We can look at correlations between measurements in different places, and see if there are patterns.

For example, if the Universe were closed, and shaped like a higher-dimensional torus, we would see identical patterns in the CMB at periodic places in the sky. The number and spacing of such patterns would tell us about the structure of the Universe on scales much larger than the present-day 46 billion light year horizon.

In fact, we don't actually see any such patterns (yet?). That allows us to infer that the Universe, out beyond the horizon, is much, much larger than what we can see (I *think* the current limit is something like a factor of 4 or 5).

Len, since the speed of light is finite, a retreating train is always a little further away than when you last saw it, since the train is moving away as the light you saw reflected off it made its weary way toward you.

If you detect or react to the train with something moving slower than light, then the difference can be huge. If you are blind and use sonar to detect the train, then it's a lot further away when you hear the echolocation bouncing off the train than it was when you sent that pulse. If you don't correct for its movement, then you'll be quite a long way off in your estimation of its distance.

Hi, just a beginner in astronomy, but I find 'filaments' fascinating...

The images that I see of filaments seem to show a certain uniformity. If we have a perspective of some 45 billon years, then should we not begin to see some 'thinning out' due to the action of dark energy?

Thanks!

No, dark energy is far too diffuse to affect such a relatively small (compared to a galaxy) object. And it can't interact any way other than gravitationally.

Thanks for your response! I guess I'm using the term (filaments) wrong, but I was thinking of that 'purple' image four from the bottom from the series shown above.

No, the purple is very rarified emission spectra gas. The UV light excites the gas. They are filaments, and they are tiny in extent and will not be affected to any visible extent by dark matter.

It's not you using the term "filaments" wrong, I knew what you meant. The response is valid for that thing you're talking about, steve,

Thanks so much for your answer! I will have to look at this more in depth - facinating!

Oh, that supercluster image? You mean that one? It's a synthetic image and inferred from positions of observed objects in 3d space. There's no actual filament there, no purple (that would, IIRC, be something like an OII line, I'd have to check up to find out).

What are you expecting to see? If there was dark matter evenly presented all over the universe on superlcuster scales, there would be no net force and therefore only the mass of the galaxy would matter, which includes the mass of dark matter in it. And the formation is the result of that force.

If the dark matter is mixed and uneven, then the position of the galaxies are the result of that inhomogeneity, and so we'd not expect any different.

So the only changes to the shape that would be unexpected would be the mass distribution of the dark matter being different from the mass of the luminous matter.

However, since they're both still ordinary matter, the original inhomogeneity would be the same for both, it's just that the visible matter collapses more readily and therefore the effect of dark matter is to slow down the movement of the visible matter, but not change its trajectory.

Steve, there's no need to gush. This is a site for asking questions. Ask the questions with displayed honest inquiry and it gets answered. "Thanks" aren't really necessary, though there's no problem with doing so. But when you gush or go over the top, you risk coming across as being sarcastic.

This is the internet. Most of the communication humans do is nonverbal and therefor not available on the internet as a text only medium. And a lot of English is in tone (nowhere near as much as tonal languages, obviously), which ALSO is incapable of being expressed on the text only internet.

Emoticons are a very limited way to insert it. As are fake tags like /sarc (because on the internet nobody can hear you Poe).

Basically, if you were asking someone you knew and liked who knew more on a subject than you, ask and thank on the internet the random joe or jane blogger the same way.

The internet is neither the home for Olympians nor the Illuminati, though many play those roles on it.