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The scale of the universe is so vast that it can be hard to grasp. One of the best ways to comprehend it is by starting on a relatively small scale with our home planet, Earth, and working outwards
at which it’s moving away from us. Hubble’s law applies because the universe as a whole is expanding, carrying distant galaxies away from each other like raisins in a rising cake.
The wider the separation between galaxies, the faster they move apart, because there is more expanding ‘dough’ between them. What’s more, this expansion stretches light from fast-retreating galaxies into longer and redder wavelengths, creating a ‘redshift’ that can be measured on Earth and used as a proxy for distance itself. Broadly speaking, the farther away a galaxy lies, the greater the redshift in its light.
Expansion and origins
Since the universe as a whole is expanding, it must have been much smaller in the past, with matter packed more closely together. Modern measurements of the rate of expansion – known as the Hubble constant – put it at around 21.5 kilometres (13.4 miles) per second per million light years of separation; in other words, on average galaxies 10 million light years apart are receding from each other at around 215 kilometres (134 miles) per second. By winding back the clock of cosmic expansion to zero, we can pin down a time when everything was in the same place: the beginning of the universe, about 13.8 billion years ago. Because matter was packed together more closely in the early universe, it must also have been hotter. In the very earliest times, temperatures were so high that any matter that formed would have fallen apart – in effect, matter and energy were interchangeable in line with Einstein’s famous equation E=mc2. However, as temperatures cooled, more complex forms of matter were able to persist for longer, eventually forming stable atoms of simple elements such as hydrogen and helium. This is the origin of the
Big Bang theory – our best explanation for how the universe began.
Cosmic time machine
An intrinsic property of the universe is that the speed of light in a vacuum is constant. Light moves at 299,792 kilometres (186,282 miles) per second – a speed that seems instantaneous in everyday life, but which is nevertheless finite – and this has important consequences for how we view distant space. In essence, the further away we look in space, the further we are looking back in time, since light must have left objects long ago to reach our telescopes now. The evolution of stars and galaxies is so slow that the millions of years taken by light from nearby galaxies makes little difference, but as telescopes and detectors have improved, it’s become possible to see objects that are many billions of light years away – so distant that we are looking back to a much earlier phase of cosmic history, when they were just beginning to form.
“more complex forms of matter were able to persist”
The first objects to be spotted at these immense distances were quasars – galaxies with monster black holes at their centres that have not yet settled down into a quiet slumber, still actively feeding on gas and dust from their surroundings. Intense light and other radiation from their central regions gives them a star-like appearance, and the surrounding galaxies only became visible as images improved. Telescopes such as the Hubble Space Telescope, meanwhile, have allowed astronomers to detect the light from fainter galaxies even closer to the Big Bang, revealing an era of chaotic mergers as small galaxies grew into larger ones such as today’s spiral and elliptical systems.
The edge of the universe
The finite speed of light puts a barrier around ‘our’ universe that we cannot penetrate. Since radiation has only had 13.8 billion years to reach Earth, it’s impossible for any information to reach us from remote parts of the universe beyond 13.8 billion light years. This defines our ‘observable universe’, a vast bubble of space centred on Earth. Objects at the very edge of this bubble are still too faint and distant for our telescopes to capture. Before the first galaxies, an early generation of truly monstrous stars were the first luminous objects to light up the young universe, and it’s hoped that some of these may finally come into view with the launch of the giant James Webb Space Telescope later this year. Beyond these lies the ‘cosmic dark age’, a period of around 400 million years after the incandescent Big Bang had cooled into invisibility, during which matter coalesced in darkness.
There's another reason for the darkness at the very edge of our observable universe. As the speed at which distant regions and objects are retreating from us gets ever more extreme in all directions, light is stretched and redshifted beyond the limits of visibility – first into the invisible infrared part of the spectrum, and then into microwaves, which are short-wavelength radio waves. The most distant radiation of all comes from the cosmic microwave background (CMB) – a faint signal that comes from all over the sky. It’s the afterglow of radiation that escaped from the incandescent fog of matter and trapped light when the universe finally cooled enough to become transparent, about 380,000 years after the Big Bang itself.
1 Earth
Earth’s diameter is 12,756 kilometres (7,926 miles). The Moon orbits Earth at an average of 384,400 kilometres (238,855 miles).
2 Solar system
The outermost planet in the Solar System, Neptune, orbits the Sun at a distance of 4.5 billion kilometres (2.8 billion miles).
3 living local
Stars in our local region of space are separated by light years – tens of trillions of kilometres. The brightest star in the sky, Sirius, is 8.6 light years away.
4 home galaxy
Our Sun and Solar System, and all the stars in our sky, are members of the Milky Way – a vast spiral of stars that’s roughly 120,000 light years across.
5 Grouping up
The Milky Way is a major member of a small galaxy cluster called the Local Group, occupying a volume of space about 10 million light years across.
6 Big cluster
The Local Group is an outlying region of our local galaxy supercluster, sometimes called the Virgo or Laniakea Supercluster. It is over 100 million light years long.
7 empty space
The Virgo Supercluster is part of a local supercluster complex a billion light years across. At this level, the large-scale structure of filaments and empty voids begins to emerge.
“the universe as a whole is expanding”