How fast is the universe expanding? To that question, much to the frustration of astronomers, we find different answers. A new trick should solve that problem.

The further away a galaxy is from us, the faster it moves away from us. Several astronomers, including the American Edwin Hubble, established this in the first decades of the twentieth century. This is due to the fact that the universe has been expanding since the Big Bang. But at what speed does it do that? There has been a lot of discussion about this in recent years: different measuring methods give different answers.

Now American astrophysicists Francis-Yan Cyr-Racine (University of New Mexico), Fei Ge and Lloyd Knox (both University of California at Davis) a way out in front of. If we manipulate the early universe in the right way and assume that so-called mirror particles exist, we can arrive at one expansion rate of the universe.

Hubble voltage

What this discussion revolves around is the so-called Hubble constant. It shows how fast a galaxy that is at a certain distance from here moves away from us. If you determine the value of this constant based on variable stars and supernova explosions, you arrive at 73 kilometers per second per megaparsec. (In other words, a galaxy one megaparsec away – more than 30 trillion kilometers – is moving away from us at 73 kilometers per second.)

But you can also get the same number from the oldest radiation we can see: the cosmic microwave background radiation. It was created about 370,000 years after the Big Bang, when the universe became transparent, and can now be seen as radiation with a temperature of -270 degrees Celsius reaching us from all directions (see the image above this post). And based on this radiation, the Hubble constant should be 67 kilometers per second per megaparsec. The difference between the two values ​​is known as the Hubble voltage.

bigger cake

Cyr-Racine, Ge and Knox now believe they have found a way to relieve that tension. According to them, you can ‘scale up’ the early universe. That is, you multiply each length and each time by a certain factor. In this way you can ensure that the Hubble constant that produces the cosmic microwave background radiation is the same as the Hubble constant determined with astronomical objects.

“Compare it to baking a cake,” says Knox. “Suppose you double the amounts of all the ingredients. Then you get a bigger cake, of course, but each bite would still contain the same ingredients in the same proportions, and therefore taste the same.”

Invisible light

Now you can’t just scale up the universe. If you do that, the density of photons – light particles – in the early universe also increases. And such a higher photon density is inconsistent with observations of the cosmic microwave background made by the FIRAS instrument on board the COBE satellitesays Knox.

He and his colleagues solve that by getting those extra photons from somewhere else. “We add so-called dark photons,” says Knox. “They are invisible to FIRAS, thus avoiding the limitations imposed on us by the FIRAS observations.”

Family of ‘dark stuff’

You will not be there with only dark photons. As mentioned, the cosmic microwave background radiation was created when the universe became transparent. The cause: before that, the universe consisted of loose charged particles, which continuously absorbed and re-emitted light. 370,000 years after the big bang did those charged particles combine to form neutral atoms, who interfered much less with the light. As a result, light could only travel much further from that moment on; the universe became transparent.

The same must then apply to dark photons if they are to play their role in removing the Hubble voltage. But then there must also have been dark charged particles at the time, which formed dark atoms that made the universe transparent to dark photons. In other words: you need a whole family of ‘dark stuff’ to make this scenario possible.

Mirror particles

Now all that dark stuff isn’t completely out of the blue. There is a lot of scientific literature about these so-called mirror particles. In fact, these are copies of the particles known to us, which we hardly notice. Only through gravity – and any new, weak forces – ordinary particles and mirror particles interact with each other. “As a result, we don’t notice any of these particles,” says Knox. “They just fly right through us.”

Such mirror particles are designed to solve other theoretical problems in physics and astronomy – and could therefore also remove the Hubble voltage. If they exist, because there is currently zero experimental evidence for that.

New path

Even if we assume the existence of mirror particles, including dark photons, we are not there yet. Just as you will have to bake a cake twice the size longer to cook it, not everything stays the same when you scale up the universe. Among other things, the amount of helium contained in such a universe does not match what we observe in our universe.

That is indeed a big, open problem, says Knox. “We – and others – do have ideas about this, but whether they work remains to be seen. We have therefore not provided a complete solution for the problem surrounding the Hubble constant, but have found a possible new path towards such a solution.”