During this whole coming week, the first part of each night is dark and moonless. It’s a perfect time to go out under the stars to contemplate the cosmos. Our unaided eyes show no trace of an expanding universe. That’s because neither our solar system, our home galaxy or our local group of galaxies is getting larger.
But the vast majority of galaxies, hundreds of billions of them, all do rush away from us, making their light redden and dim. This arises because space is stretching. That lets distant galaxies race away faster than the speed of light.
Granted, big bang cosmology is a disquieting and non-intuitive model for our universe, its contents and its destiny. It contains many surprises which have yet to be worked out. Regardless, we should picture galaxy clusters as basically static objects increasing their distances solely because the empty space between everything is inflating. And we mustn’t imagine the cosmos expanding into some mysterious primordial void which is itself empty of space, time or matter. No such place exists, since there is no “outside” to the universe.
Nor can any notion of an “outside” be answered by multiverses for which, incidentally, there is no shred of evidence. To picture what’s beyond doubt, simply imagine that wherever you live in the universe, everything that’s not nearby is racing directly away from you. Since this increasingly dims the light from distant galaxies, it produces a cut-off, a blackness that lies 13.8 billion light-years away.
We cannot see anything beyond this point, and never will, because light from there will never arrive. There is thus a specific edge to the visible universe. This lets us talk about the contents of the observable cosmos, meaning everything nearer than that. We can specify its total weight or mass, determined by its overall gravitational effect. We can say the number of atomic particles (protons, neutrons, etcetera) in the universe is a 10 followed by 88 zeroes.
This material is mostly hydrogen atoms, a gas. There are also some solids, mostly dust specks the size of cigarette smoke particles. The absolute most common particle is invisible: neutrinos, which are everywhere, flying around just below the speed of light. Every second, four trillion neutrinos zip through each of your eyeballs. Nothing stops them because they do not interact with matter. After being created in the core of the Sun and most other stars, they each zoom entirely through the Earth in 1/20th of a second and then through your body.
A billion times more plentiful than neutrinos are the massless bits of light called photons. Since mass and energy have equivalence, photons count when we appraise the weight of the universe.
All of this — neutrinos, dust, photons and stars — are invisible beyond a distance of 13.8 billion light-years from Saugerties. Leading us to wonder about that other cosmos, the bigger one that lies beyond the observable universe. Strong evidence from 2013 shows that it doesn’t warp our own space and time, which means it’s enormous. It may even be infinite in inventory and in physical extent; that notion is steadily gaining traction among cosmologists.
We cannot really say anything definite about that real universe. This creates an awkward moment whenever someone asks on public radio, “How many galaxies are in the universe?” and I must reply, “Which universe are you talking about?”
Because the vast majority are unaware that there are two universes — the observable one and the much larger one that lies beyond. Indeed, many further compound the confusion by assuming that multiverses exist and must be reckoned with too. Actually, multiverse is just an idea, a notion with no observational basis, a way of trying to explain certain aspects of the cosmos. And a conversation stopper, since there’s nothing substantive anyone can say.
Wrapping this up, we live within two universes, only one of which we know anything about. And even that one with its bewildering supposed origin as an ultradense pea-sized ball that popped out of nothingness leaves our heads spinning. But at least we have an exact figure for its size, which by itself is a lot to chew on.
This page has been lately exploring the strange and largely unknown nature of the universe considered as a whole. Starting next week we’ll get into the nearby fun stuff in the night sky, which generally makes our heads spin in a more familiar and comfortable way.