We all know that what we think of as the vacuum of empty space is actually anything but “empty”. Even a hard vacuum contains a low density of particles along with dust, electromagnetic radiation and so on. Interplanetary space within our own Solar System includes around five particles per cubic centimetre at the Earth’s distance from the Sun, decreasing as you go further out.
And, quite obviously, matter is distributed unevenly throughout the universe, with galaxies, clusters of galaxies and superclusters being spread around in a seemingly random fashion. This probably results from uneven conditions that go right back to the initial stages of the Big Bang, for which purpose Steven Weinberg’s 1977 description The First Three Minutes: A Modern View of the Origin of the Universe (updated with a second edition in 1993) is still one of the most readable summaries.
Professor Weinberg takes us through the process of the initial expansion from which the current complex spread of material developed. So, over the intervening 13.8 billion years, our Milky Way Galaxy has ended up, as Noam I Libeskind and R Brent Tully describe in Our Place in the Cosmos (Scientific American, July 2016), amongst around fifty other galaxies in the Local Group, located just outside the Virgo Cluster of over 1,000 galaxies, within the vast Laniakea Supercluster spread across 400 million light-years. With plenty of gaps of so-called “empty” space around and in between, some areas of space being significantly more empty than others.
Poul Anderson’ 1970 novel Tau Zero, which was nominated for a Hugo Award, considers the concept of just how “empty” space may be as a key element of the plot. A manned interstellar spacecraft, based on a Bussard ramjet design, unexpectedly runs through a nebula when moving at a substantial percentage of light speed, damaging the scoop that was to have decelerated the vessel. The only solution is to accelerate close to the speed of light in order to leave our Galaxy entirely and find a place where space is sufficiently empty to allow the crew to make repairs safely whilst travelling at such immense speed.
However, even intergalactic space is not empty enough. Eventually they find that only in the space between superclusters of galaxies is the tiny amount of matter that exists there spread thinly enough for repair work to begin. But then comes the additional problem of finding enough matter available in such a remote region to permit the spacecraft to decelerate…
It looks as though there may actually be some areas of space where matter is spread extremely thinly indeed – as close as we’re ever likely to get to truly “empty” space. In The Emptiest Place in Space (Scientific American, August 2016), István Szapudi of the University of Hawaii’s Institute for Astronomy describes how a giant supervoid, some 1.8 billion light years across, has been identified only three billion light years from Earth, making it still relatively close in terms of the scale of the universe as a whole.
Dr Szapudi points out that this supervoid may be “the largest structure ever identified by humanity” (which may tend to go against our everyday use of the word “structure”), though current theory suggests that there should only be a few more of them within our observable universe.
It’s intriguing to think of such vast areas of emptiness, with a total absence of any of the stars and galaxies with which we populate our imaginings of space travel. A Bussard ramjet, if such a thing ever becomes feasible, would undoubtedly find difficulties there. But the view (or rather, in a sense, the absence of it) would be quite extraordinary. A bit monotonous, but well worth exploring.
Richard Hayes, Assistant Editor (Odyssey)