Energy Matters

In Dark Energy by Brian Koberlein3 Comments

Yesterday while doing an AMA on Reddit I was asked an interesting question.  How do we know that dark energy, which is the attributed cause of cosmic expansion, isn’t just gravitationally repulsive matter?  After all, electric charges can attract and repel, so why not masses?  The repelling masses would push against each other and expand the universe.  How do we know that isn’t the cause of dark energy?  The short answer is that the observational evidence doesn’t support the idea.  But it is an interesting demonstration of how you can build and test ideas.

Let’s start with the idea that mass is somehow similar to electric charge.  In electromagnetism, there are positive and negative charges, and they follow the rule that charges of the same kind repel each other, while opposite kinds attract.  With gravity, masses of the same kind attract each other, so we could suppose that there is positive and negative mass, where masses of the same kind attract each other and opposites repel.

With charge, opposite kinds tend to come together to form objects that are electrically neutral.  The atoms in our body, for example, have positive and negative charges, but they tend to be distributed pretty evenly.  For our hypothetical positive and negative mass, you would tend to get clumps of one kind or another.  So we might speculate that some galaxies are formed of positive matter, while others are formed of negative matter.

Galaxy clusters in the universe. Credit: Sloan Digital Sky Survey.

Galaxy clusters in the universe.
Credit: Sloan Digital Sky Survey.

That’s an interesting idea, but if that were the case, positive mass galaxies would attract other positive mass galaxies, but repel negative mass galaxies.  The same would be true for negative mass galaxies, so we would expect to see galaxies tend to differentiate into clumps of different mass types.  What we actually observe is that galaxies always attract each other.  The clustering of galaxies is consistent with them always attracting each other gravitationally, and there is no evidence of a differentiation.  So that idea doesn’t work.

So let’s try another idea.  Two types of mass, one being the regular kind that gravitationally attracts all other masses, and a second kind that always repels all other masses.  At first blush this might seem to be a good idea.  Regular matter would clump into galaxies and clusters, just as we observe, and this new repulsive matter would tend to spread out evenly and push against all the galaxies.  So you would get clumping and cosmic expansion just like we see.  Right?

_61001522_hist_universe464Not quite.  The good news is this idea makes a clear prediction.  If cosmic expansion is due to some repulsive matter, then as the universe expands the density of that matter would decrease over time.  If the repulsive matter is more thinly spread, then its effect would be lessened.  So the clear prediction is that cosmic expansion is slowing over time.

This isn’t what we see, so this idea doesn’t work either.

What we actually see is that the universe is  expanding at an exponential rate.  This means that the rate at which the expansion occurs doesn’t decrease.  It is constant.  As the universe expands, the density of dark energy doesn’t decrease.  In general relativity this means that dark energy is a property of spacetime itself.  It cannot be some form of repulsive matter because the effect of cosmic expansion is persistent and unchanging.  Observations of distant galaxies confirm this.

So while we aren’t entirely sure what dark energy is, we do know unequivocally that it is not matter.


  1. As an uneducated fan of physics, it’s hard to stop building more fanciful and less likely possibilities to explain these mysteries. E.g., “What if these particles are being created constantly, either in or out of galaxies, eliminating the dispersion effect?” “Observational experiences wouldn’t show these because, due to their nature, we would be as likely to run come in contact with them as a cumulus cloud would be to come in contact with an anvil.” “How much weight can we give the assumption that these other particles would be similar to known particles? ”

    Meta-question: Does the knowledge foundation you’ve built over your years of study and practice help you know when thinking outside the box is way too far outside the box? Because reading science fiction sure doesn’t.

  2. I have thought about this idea too, and looked into it. Formerly repulsive gravity was not considered either, neither as negative energy (gravitational energy is negative), but in inflation theory, they are part of the foundation of that theory. Along that line one could think: why couldn’t mass also not exist in both positive and negative form?
    But then you already have two different kind of negative mass you can think of, namely either assuming that gravitational mass equals inertial mass (like we ordinary do) or the opposite, namely that inertial mass is equal to the absolute value of gravitational mass (or, which is equal, the negative of the gravitational mass).

    This then gives for the properties of such masses in gravitational field:

    For assumption 1., the properties of negative mass are:
    – negative mass attrackts negative mass, but then (F=ma, inertial mass = gravitational mass) accelerates opposite to the force! Effectively then negative mass self repels.
    – positive mass is repelled from negative mass.
    – negative mass accelerates towards positive mass.

    For assumption 2., the properties of negative mass are:
    – negative mass attracts negative mass (and here: also accelerates in the same direction)
    – positive mass is repelled from negative mass (as in assumption 1.)
    – negative mass is repelled from positive mass.

    But then note that assumption 2. is in fact unnecessary, because all that it does is interchange the concept of positive and negative, and therefore it would be just the same as negative mass matter in assumption 1. The type of mass that self attracts we then could call the positive mass, the mass that self repells the negative mass.

    This then is different with your idea that negative mass would still attract. In fact it DOES attract, but the acceleration is due to the negative mass OPPOSITE to the force!!

    Now then the dynamics are totally different as in your assumptions! Let’s look at it.

    Positive mass matter would as in the big bang model form galaxies and stars. Negative mass matter however self repells and spread out. But since it accelerates still towards this positive matter clumps, it will surround the galaxies, and have influence on the rotation speed of the galaxy. Since it also self-repels, it would therefore not exist inside the galaxy but only around it.

    Another property is the electromagnetic properties of negative mass matter. Here the negative mass matter inverts attraction and repulsion as compared to positive mass matter. Positively charged negative mass matter will attrackt positively charged negative mass matter, that is like charges attrackt, unlike charges repell. For electromagnetic interactions between positive mass matter and negative mass matter, we can also find the interactions for the combinations of the two types of mass and two types of charge (which here for brevity I have left out, but which can easily be constructed, just by using the conventional physical laws, but allowing also negative values for mass).

    Now this is of course purely theoretical, and noone seen ever negative mass matter. Standard physics currently excludes the negative mass concept, and considers it unphysical. But then nobody ever saw dark matter either, in fact we still don’t know what it is, we can only say what it is NOT. It might be that negative mass is the only remaining candidate and can be shown to fit the data quite well. That is of course something that some smart physicist knowing the properties that dark matter must have, must be able to sort out. Maybe the idea then can be shown to be impossible to fit there, but if it does, then something fundamentally different as we until recently thought might be the case. With would have huge impact on all of physics laws which concerns the mass concept. It then could probably also, due to the symetry, a way of unifying gravity with the other forces, which until now, we can’t.

    Then we must put it to the test and to see if all our data about the universe can be fit onto this new understanding.

    I have no idea if that can be the case, but it can’t be ruled out a priori I guess.
    But it is quite some paradigma shift if we could fit a connection between dark matter (and possibly also dark energy) and the negative mass concept. Untill that has been sorted out, it is just that: a wild speculation.

  3. I made my post too early, in fact you are right that the two types of matter (the two assumptions I made in my earlier post) are distinct of each other and not the same. On itself then, I agree, it can not explain dark energy. But maybe dark energy exists also as a vacuum energy, which as space expands, creates more repulsive force (due to the energy density staying the same in space, and as space expands, the nett effect is more repulsion/expansion) and the total repelling force driving the universe expansion is the sum total of both the vacuum energy and the negative mass.

    Still I would think there might be a case for negative mass for explaining the dark matter. Or can someone make a clear case against it?

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