Peter Eisenhardt

WISE project scientist

Do clusters and superclusters act as a single entity?

Groups are gravitationally bound. Then you have larger clusters a magnitude of ten bigger than groups, the largest gravitationally bound structures. When we say gravitationally bound, I mean bound in the same way Earth and the planets are bound to the Sun, except, there’s not really a central, dominant equivalent of the Sun.

Can superclusters get bigger than the LQG?

The distribution of galaxies is not the same if you look along the distance between here and the Coma cluster – 300 million light years. The universe is lumpy [unevenly distributed] on that scale, but we know there’s not much lumpiness on a factor of ten larger than this scale, because we’ve been able to probe out for much larger distances than 300 million light years. We can see that on the scale of a billion light years, the universe is pretty much the same no matter where you look. An important point is that the larger structures have been at the centre of the realisation that most of the gravity is coming from dark matter. And they’re important for understanding the history of the universe because the size of these structures tell us what has happened over the history of the universe.

Between the clusters are voids: are those truly empty?

I would hesitate to say there’s nothing in them. Voids are substantially under-dense. Every[where] in the universe today there is ionised hydrogen – protons and electrons. The density of that ionised hydrogen doesn’t vary tremendously, so in the voids, it’s not vastly less dense than in the clusters.

What’s ‘big’ for a cosmologist?

300 million light years isn’t an instant, but it’s more or less contemporaneous. A billion light years is starting to be interesting. The nearest star being four light years away is still an awfully long way. It’s mind-boggling how much we know having not gone the tiniest fraction of that distance.

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