“We are just an advanced breed of monkeys on a minor planet of a very average star. But we can understand the Universe. That makes us something very special.” -Stephen Hawking

You’re probably familiar with the standard picture of our Universe. You’ve heard it all before: that the Universe we know of — stars, planets, atoms, etc. — is less than 5% of the Universe’s total energy. That most of the matter is dark matter, and that most of the energy in the Universe isn’t matter at all, but dark energy.

But recently, we’ve started to discover a couple of interesting things about the atoms in the Universe. First off, take a look up at the night sky, and you’ll be greeted by a familiar sight.

Image credit: Chris Hetlage.

Stars! Our galaxy — like all galaxies — is full of them, as you can see by looking at M46 and M47, above. We’ve known for a long time that there aren’t enough stars in the galaxy to explain what we see gravity doing, so we know that most of the mass in the Universe isn’t stars.

At least, it isn’t conventional stars. But you might start to wonder, what if there are lots of small, low-mass, very dim stars out there? What if, in fact, there are failed stars out there?

Image credit: European Space Agency.

Not just dim, red, M-stars, which still fuse hydrogen into helium, as long as they’re about 1/12th as massive as the Sun. But even smaller, lower mass ones. Maybe they can fuse deuterium, which makes them brown dwarfs, or maybe they’re just big, Jupiter-like blobs on their own. Regardless, this could be considered some type of “dark” matter, because we don’t see it with standard telescopes.

However, there is a very clever way to detect such objects.

Image credit: STScI.

When one of these “rogue planets” passes in between us and a background star, we’ll see that star briefly brighten and then dim again, thanks to a process called gravitational microlensing. While searches such as MACHO and EROS showed that these objects can’t be most of the missing matter, they can still be a significant amount.

Image credit: Jon Lomberg.

And recently, a team has found many more of these planets — freely floating through space and not attached to any star — than we thought! Again, it isn’t enough to be all (or even most) of the dark/missing matter, but it’s something!

We can also look at other things that have mass: things that aren’t stars, planets, or other collapsed objects. Things like interstellar gas and dust, like Bok Globule B68.

Image credit: European Southern Observatory.

After all, if there’s plenty of gas and dust, maybe that could be some of the dark matter, too! In fact, it’s just been discovered that there’s plenty of this, too, in the Universe. It’s actually really cool. When we look far out in the Universe, we can map out where the galaxies we can see are.

Image credit: 2dF galaxy redshift survey.

You’ll notice that the shape of this looks like some type of web, or network. (Someone has even pointed out to me that it looks a lot like a series of neurons in the brain!) If we try to simulate structure in the Universe, we get something that matches observations (and neurons) very well.

Image credit: Mark Miller, Brandeis Univerity; Virgo Consortium for Cosmological Supercomputer Simulations.

If you look at the “nodes” above, that’s where you’re going to find the greatest concentrations of galaxies clustered together. But if you look between the nodes, along the imaginary lines connecting them, you’ll find a few, small galaxies, sure. But you’ll also find X-rays, which come from the collapsing gas clouds!

Animation courtesy of In The Dark.

So, if there are more rogue planets than we thought, and more dim stars than we thought, and more intergalactic gas and dust than we thought, is it possible that we don’t need dark matter? Or, a little more conservatively, is it possible that we need less dark matter?

There’s only one way to decide: let’s ask the Universe! We can look at cosmic structure formation, above, as well as…

Image credit: WMAP Science team and NASA.

The fluctuations in the Cosmic Microwave Background, and…

Image credit: MAP990403, taken from UIUC's website.

The primordial abundances of the light elements: Hydrogen, Helium-3, Helium-4, Deuterium, and Lithium.

These are observations we can make that tell us how much “atomic” matter there is — stuff made out of protons, neutrons, and electrons — versus how much is truly some new type of matter that doesn’t emit light.

And all of these observations — these independent observations — point to the same thing: a Universe that’s about 4.5% atoms.

Image credit: Physics for the 21st Century.

Not only can we not get rid of dark matter, we can’t even make a dent in it!

But then, what do these extra planets, dim stars, or gas mean for our Universe?

The truth of the matter is, they simply tell us how that 4.5% of atoms is divided up.

Chart courtesy of Fukugita and Peebles, 2004.

It’s important and fun to know how the normal matter in the Universe is divided up, and how much of our Universe is made of stars, planets, gas, dust, or anything else you can think of, but no matter how it’s divided, you can’t replace dark matter with it.

Too many things would be different. Large-scale structure would be all wrong; you’d see too much Silk damping. Nucleosynthesis would be all wrong; you’d have too much helium and too little deuterium. And the fluctuations in the microwave background would be all wrong; the third peak wouldn’t be there.

It’s why we do the measurements we do, and this is what we learn from them: physical cosmology requires a Universe with 20-25% dark matter, and with just 4-5% of normal (atomic) matter. And what an interesting thing to learn: no matter how the 4.5% of the Universe that’s made out of atoms is split up, we still need just as much dark matter to make the Universe the way it is.

Comments

  1. #1 Remo
    May 25, 2011

    So, let me get this right: the Wimps of the universe continue to beat up Machos

  2. #2 islami chat
    May 25, 2011

    Evrensel değerleri ve fikri hakları koruyan minimum sınırlama getirilmeli ” şeklindeki Fransa Cumhurbaşkanı Nicolas Sarkozy’nin önerisine karşı sektör temsilcileri ise, “mümkün olduğu kadar sınırlamaktan uzak durun’ görüşünü savunacak

  3. #3 AJKamper
    May 25, 2011

    So, what am I missing?

    1) Dark matter was hypothesized in order to fix galactic rotation velocities, since the amount of visible matter didn’t explain the high velocity.

    2) The amount of dark matter suggested by the CMB and relative abundance of elements was in line with the amount necessary to fix those galactic rotation velocities.

    3) Now we have discovered a chunk of matter which would also help account for the velocity.

    4) Doesn’t this imply that the amount of dark matter predicted by other sources is now out of whack with that necessary to fix the predicted rotation of galaxies?

  4. #4 SLC
    May 25, 2011

    The apparent fact that there may be hundreds of billions of these rogue planets wandering around our galaxy is an amazing discovery, as unexpected as the discovery of dark energy.

    This has been noted on other blogs and has revived interest in a science fiction novel published in 1933 entitled, “When Worlds Collide,” by E. Balmer and P. Wylie.

    The plot of the novel is that a planet about the size of Neptune is discovered with a satellite about the size of the earth heading toward the solar system, by an astronomer named Bronson. The larger planet is denoted Bronson Alpha, the smaller Bronson Beta. Computations indicate that Bronson Alpha will collide with the earth (remember, this was written in 1933 before the invention of computers), which will detach Bronson Beta and send it into orbit around the sun between Mars and Venus. Then a frantic race to build space ships capable of transporting people to Bronson Beta begins in the US, Great Britain, France, Germany, and Russia. Eventually, 2 ships carrying 1000 people from the US, a British ship, and a Russian ship successfully make the trip. There was also a movie made in 1951 based on the novel.

    At the time, the novel was considered very far fetched indeed because astronomers were totally unaware of the existence of extra solar planets. A remake of the film is planned for 2012 by the Dreamworks Studios, which also made the remake for “War of the Worlds”. This discovery will certainly provide interest in the film which is now not at all far fetched.

  5. #5 Wow
    May 26, 2011

    It would be interesting to hear an answer to question #4 by AJKamper.

    And given that dark energy is there because we see “deep time” galaxies behaving as if there were less gravitating matter (including dark) in the nearer bits to us, there does seem to be rather an obvious suspicion of “circular reasoning” going on:

    1) we need more mass, dark matter
    2) we need less mass, dark energy

    but if we find more dark energy, we need more dark matter and if we find more matter, we need more dark energy and more dark matter.

    Or maybe we shouldn’t be adding in more dark matter in the first place…?

    I wonder if, to those ancient (young) galaxies (the observation we have of them are when they were in a very young universe), there was just less gravitational pull. Continuous creation could cause that appearance. So could gravity changing over billenia. And if there were inflation, if it were still happening a little at that time, there would be more matter visible to them near us than we see looking toward them in the past. That distortion in time could make us “see” more matter than that galaxy “saw” when it shed those photons and fitting what we “see” now to what it “saw” makes it look like there’s less mass to the universe in the past.

    Time travel in english is hard.

  6. #6 Bjoern
    May 26, 2011

    @AJKamper:
    Concerning your point (1): that is right – but you miss something important there: it has also been known for a long time now that the total amount of baryonic matter (including “invisible” matter!) didn’t explain the high velocity in the rotation curves. That amount had been determined e. g. from the rations of helium, deuterium etc., as mentioned by Ethan above.

    Concerning your point (3): the newly-discovered planets don’t have enough mass to explain the rotation curves – by far not enough mass!

    Concerning your point (4): No, not at all. The amount of dark matter which is necessary has been calculated by measuring how much baryonic mass there is in the universe – not simply by looking how much visible mass there is. And the total amount of baryonic mass hasn’t changed in any way due to the newly discovered planets; as Ethan explained, the only new thing now is that we now know more about of which things the baryonic mass consists – besides the obviously visible parts.

  7. #7 Bjoern
    May 26, 2011

    @Ethan: In the table taken from Fukugita and Peebles, what does “substellar objects” refer to? Brown dwarfs? Planets apparently aren’t included there. (- or are they?) And what is meant by “condensed matter”? The “rocky” planets like Earth, or what?!?

  8. #8 AJKamper
    May 26, 2011

    @Bjoern:

    I think your argument is saying two different things. That is, we need to figure out the ratio of baryonic matter to dark matter to make the CMB and nucleosynthesis work, but I’ve never heard that we could figure out the amount of baryonic matter in a distant galaxy just by looking at it. Zwicky, for example, was just noting that the amount of visible mass and mass we could account for by dust and so on didn’t add up. In other words, the galactic rotation still left open the possibility of baryonic matter making up the missing mass… as far as I’ve ever heard, at least, though I certainly could be wrong.

    At the same time, if the mass of interstellar planets and gas ain’t enough by an order or magnitude or more, then you’re right; it doesn’t cause a significant problem.

    But they’re two different problems that I’d like to make sure aren’t conflated.

  9. #9 bjoern
    May 27, 2011

    @AJKamper:

    Zwicky, for example, was just noting that the amount of visible mass and mass we could account for by dust and so on didn’t add up. In other words, the galactic rotation still left open the possibility of baryonic matter making up the missing mass… as far as I’ve ever heard, at least, though I certainly could be wrong.

    Yes, if you only look at rotation curves, the missing matter could be baryonic matter. But since we not only look at rotation curves, but also at other observations, that point seems moot…

    At the same time, if the mass of interstellar planets and gas ain’t enough by an order or magnitude or more, then you’re right; it doesn’t cause a significant problem.

    Look at the table above (from Fukugita and Peebles, 2004). Planets (the ones in solar systems) make up only 10^(-6). Even if you add several hundred billion planets (outside solar systems), you obviously won’t get more than, say, 10^(-4). That’s way too small to account for the whole missing 0.22!

  10. #10 Wow
    May 27, 2011

    “But since we not only look at rotation curves, but also at other observations,”

    Such as…?

    PS why only several hundred billion planets? the mass that is insufficient to make a star can make lots of planets.

    If the gas cloud is accretes into too many centers, then, just like an overabundance of CCNs in a cloud chamber, there are no large accretions because there’s not enough spare to create a large accretion, even though there’s enough water to make a big lump of water in the chamber.

    Incredulity isn’t the answer.

    If the case for a “normal” hydrogen cloud would be that this case was so unlikely as to be impossible (by looking at the dynamics, not by going “no way”), then that is an answer.

    And it isn’t necessary to account for the whole missing .22. It only has to account for 0.01 and then it’s reducing the account of dark matter.

    Nobody said that there was no need for it.

    Just pointing out the oddity that there seems to be some circular reasoning going on here: We have to have more dark matter because we have even more dark energy if we have more non-dark matter, therefore we’re not reducing the proportion of matter.

    Go back and prove the “We have to have” at the beginning.

  11. #11 Wow
    May 27, 2011

    “it has also been known for a long time now that the total amount of baryonic matter (including “invisible” matter!) didn’t explain the high velocity in the rotation curves. ”

    IIRC, that’s false.

    There isn’t enough baryonic matter visible to us to explain the rotation curves of the galaxy, but that is less a concern.

    We also “need” dark energy to explain the retreat deceleration (or rather lack of) of galaxies at high Z values. NOT their rotation.

    But with Dark Energy there, we need even more dark matter to explain the rotation of nearby galaxies (which would have dark energy “hiding” some of it’s matter visible as gravitational motion).

  12. #12 Bjoern
    May 27, 2011

    @wow:

    “But since we not only look at rotation curves, but also at other observations,” Such as…?

    These have been mentioned both by Ethan above and again by me in the comments. Look it up yourself!

    PS why only several hundred billion planets?

    As Ethan mentioned, there was a study done recently which looked for such free-floating planets. From the results of that study, one can estimate that there are probably around 100 to 400 billion free-floating planets in our galaxy.

    Just pointing out the oddity that there seems to be some circular reasoning going on here: We have to have more dark matter because we have even more dark energy if we have more non-dark matter, therefore we’re not reducing the proportion of matter.

    That’s a straw man – nobody here used that argument. (and frankly, I don’t even understand exactly what you are trying to say here…)

    “it has also been known for a long time now that the total amount of baryonic matter (including “invisible” matter!) didn’t explain the high velocity in the rotation curves. ” IIRC, that’s false.

    No, that’s true. As Ethan explained in this and in many other articles already.

    We also “need” dark energy to explain the retreat deceleration (or rather lack of) of galaxies at high Z values. NOT their rotation.

    The arguments here have nothing to do with dark energy. Another straw man?

    But with Dark Energy there, we need even more dark matter to explain the rotation of nearby galaxies (which would have dark energy “hiding” some of it’s matter visible as gravitational motion).

    And again, I don’t understand what this is supposed to mean. What has dark energy to do with the rotation of galaxies? In what way should dark energy “hide” some of the matter?!?

  13. #13 AJKamper
    May 27, 2011

    @Bjoern: Okay, hypothesize that the total mass of the planets found WAS of such a magnitude that it could substantially explain the rotation curves. If that’s the case, then the idea of “dark matter haloes” around galaxies would be in serious trouble. Whether or not other sources found a reason to believe in dark matter, we’d have to seriously start asking where this stuff was, if it weren’t responsible for galactic curves. You could hypothesize that it was evenly spread throughout the Universe, but then the Bullet Cluster lensing would fall apart. In short, it would be a HUGE problem. (Good thing this hypothetical isn’t true, huh?)

    @Wow: I’ve never heard anyone say that dark energy is only necessary because of the dark matter hypothesis. I think, in fact, that this is completely wrong. Where did you get that idea?

  14. #14 Bjoern
    May 27, 2011

    @AJKamper:

    Okay, hypothesize that the total mass of the planets found WAS of such a magnitude that it could substantially explain the rotation curves. If that’s the case, then the idea of “dark matter haloes” around galaxies would be in serious trouble.

    Not only that idea – a lot of other observations would be contradicted by that!

    In short, it would be a HUGE problem. (Good thing this hypothetical isn’t true, huh?)

    Well, yes. But if you agree that this is only hypothetical and not true – why do you keep harping on this?

  15. #15 AJKamper
    May 27, 2011

    @Bjoern

    Well, yes. But if you agree that this is only hypothetical and not true – why do you keep harping on this?

    Because you didn’t seem to be understanding the nature of my concern, since you kept mixing it in with the CMB/nucleosynthesis observations. So I wanted to make sure you did.

    But now you do! So all is well.

  16. #16 daedalus2u
    May 28, 2011

    If planets and other non-luminous baryonic matter make up a significant amount of mass in galaxies, they still don’t explain the anomalous rotation because they would not form isotropic halos, they would be in the plane of galactic rotation.

    Baryonic matter is in the plane of galactic rotation because it interacts with other baryonic matter and out-of-plane rotation gets attenuated and kinetic energy gets dissipated. Dark matter is distributed isotropically because it doesn’t interact except through gravitation. Dark matter can be scattered via gravitational interactions but the kinetic energy isn’t dissipated so it remains in an isotropic spherical halo.

  17. #17 jirsay
    June 1, 2011

    Fascinating article. I need to read up more on this subject.

  18. #18 OKThen
    June 2, 2011

    It’s just been discovered that there
    1) are more rogue planets (i.e.unattached to stars) than previously thought
    2) is more interstellar gas and dust than previously understood
    BOTH are excellent science.

    But the dark matter hypothesis remains unshaken. Well the dark matter hypothesis has proved itself an excellent focal point motivating experimental, observational and theoretical work. A scientific focal point,like the Bermuda-triangle, is an important area of consistently crazy phenomena with a big sign that says, “Researchers explore here!!”

    Excellent scientific focal points serve science even if the underlying hypothesis fails, e.g. turn of the last century, the aether was an excellent scientific focal point.

  19. #19 JonD
    June 14, 2011

    It’s interesting that as great as science is, we don’t know what makes up 96% of what’s out there.

  20. #20 nikhil reddy
    auckland, nz
    October 20, 2013

    can i please use the image of the stars for a school assignment?

  21. #21 Wow
    October 20, 2013

    You should check on the credit for the image to find out if it can be freely used. I believe they are, but best to check. You will need to give a citation for the image too so that others can find out where it came from and see what else is there.