The cosmologists are still speculating, AND still making measurements. The measurements of the moment seem to indicate an accelerating expansion of the Universe; future measurements (which might encompass additional factors not-yet-thought-of) could possibly indicate a different conclusion. We shall see. At the moment therefore, your guess might be as good as anyone else's.

So, here is a guess, first proposed by the late

Dr. Robert L. Forward [nofollow]. It starts with the Question of, "Where did the mass-energy of the Universe come from, without violating the Law of Conservation of Energy?". The proposed answer is that the Universe consists half of ordinary mass-energy and half of "negative" mass-energy. This stuff is not impossible in General Relativity; it would simply be associated with "curved space" that has the opposite curvature than that which is associated with ordinary mass-energy. Negative mass-energy would also have a set of properties that, compared to itself, could be very similar to the properties of ordinary mass-energy (compared to itself). It could gravitationally attract itself, for example, but showing

**why** it might be able to do that will require a bit of explaining.

**First**, though, let me make very clear that the stuff known as "

antimatter [nofollow]" cannot possess negative mass-energy. That's because physicists on Earth have been making lots of particles of antimatter for decades, and

**always** have been making them from ordinary energy, per Einstein's famous equation E=mc

^{2}.

*Half* of that ordinary energy became ordinary matter, and the other half became antimatter. Therefore antimatter consists of ordinary energy, period. (Note this means that if matter can exist having negative mass-energy, it might be able to exist in two forms, one being the equivalent of ordinary matter, and one being the equivalent of antimatter. I'm only going to talk about the ordinary-matter equivalent, below.)

The explanation begins with

Planck's Constant [nofollow] and the equation that describes the energy of an ordinary photon (the first one on that linked page). Because the energy of an ordinay photon is positive, Planck's Constant is associated with positive energy, per the rules of "

Dimensional Analysis [nofollow]". This means that if we want to write the equation that describes a photon that has negative energy, Planck's constant, in that equation, must also be associated with negative energy (it must be a negative number, not a positive number).

*This is a very important point.* Most of the time, when physicists have deigned to speculate about negative mass-energy, they (including Dr. Forward) have failed to include that point, about using a negative Planck's Constant in their speculations. The net result is that they would say that when interacting gravitationally, two objects having negative mass-energy would exhibit acceleration-away-from-each-other. However, if we are ever one day to describe Gravitation using Quantum Mechanics, at that time Planck's Constant will become an integral part of that description (most probably buried somewhere inside "G", the

gravitational constant [nofollow]). This means that if we use those Q.M. equations to describe negative mass-energy and its behavior, we will need to use the negative version of Planck's Constant (and therefore a negative "G"), for those equations to be self-consistent. And one result will be, two objects having negative mass-energy will exhibit acceleration-toward-each-other, just like two objects having ordinary mass-energy.

The fun begins when we start to think about gravitational interactions between one object that has ordinary mass and one that has negative mass. Which version of Planck's Constant should we use? I have no idea (maybe both!)! There is a chance that the two objects would exhibit acceleration-away-from-each-other.

*If so*, then,

*and if* half of the Universe consisted of negative mass-energy, per Energy Conservation, then we could expect the ordinary mass to accelerate toward itself, the negative mass to accelerate toward itself, and the two different types to accelerate away from each other.

*The net result would look a lot like today's Universe [nofollow].* The obvious "clumps" are places where ordinary mass has accumulated, and the "voids" are the places where negative mass has invisibly clumped (our instruments are not designed to detect photons of negative energy, and that's why the "voids" look like voids, despite the presence of superclusters of galaxies of stars that are fusing "negative hydrogen").

It remains an interesting question, regarding the Universe as a whole, whether or not the current distribution of ordinary and negative matter can result in an overall increased expansion. Remember all the ordinary matter is attracting itself, and all the negative matter is acting equivalently. This should balance any repulsion between the two. So, we could not expect an overall acceleration. On the other hand, we

*also* could not expect the original rate of expansion of the Universe (after the "

inflation [nofollow]" period, I'm talking about here) to have ever slowed down! This is why it is critical to obtain more observational data. The observed speeds of galaxies do not necessarily need to be explained in terms of gravitational slowing (due to a Universe of ordinary mass only), followed by as-yet-unexplained acceleration. The matter in the Universe could have been going that fast all along.