Hubble helped to reveal the size of our universe
There are several candidates for the most impressive astronomical photographs ever taken.
For example, the first photograph of a person on the Moon (1969); the first picture of the Earth taken from the surface of the Moon (1969); Mars Rover Curiosity’s photograph of the distant earth taken from the surface of Mars (2014); the detailed photographs of the planets and some of their moons captured by the Voyager 1 and 2 spacecraft as they flew by, and the Pale Blue Dot photograph (1990) of a tiny Earth taken by Voyager 1 at a distance of 6 billion kilometres, 13 years after being launched, on its way out of the solar system.
But when it comes to better understanding the origin, size and age of our Universe, the most important astronomical photograph is that taken by the Hubble Space Telescope (HST) Deep Field North optics in 1995. Ninety-nine 99 per cent of the light sources you see when you look skywards are stars in our galaxy, the Milky Way, but with powerful telescopes it is possible to see other galaxies far beyond our own.
Because light travels at a finite speed, 300,000 kilometres per second, it takes time for the light from any celestial object to reach Earth.
The light from the nearest substantial galaxy to Earth, the Andromeda galaxy, takes 2.5 million years to reach us – this means we see this galaxy as it was 2.5 million years ago.
Prior to 1995 the HST had captured images of galaxies, small dots of light distributed more or less evenly across the photograph but with substantial regions of dark, empty space between them.
Then, in 1995 the HST team decided to concentrate the Deep Field North optics on a dark part of the sky.
The idea was to try to capture light from one or two very faint, very distant galaxies.
The region selected was a tiny fraction of the night sky, the equivalent area of a grain of rice viewed from nine metres away. The telescope took 342 exposures of this region between 18 and 28-December 1995. The results shocked the team.
The super sensitive Deep Field instruments showed that far from being empty, the region contained over 3,000 galaxies, some very faint.
Several of the galaxies were so distant that the light from them had taken approximately 13 billion years to reach the Earth, that’s just under one billion years after the Big Bang. It may have been that the Deep Field North region was special. To check if this was the case, the optics were directed to a different region of the sky. This experiment was the Hubble Deep Field South.
The photographs showed similar results to the Deep Field North, proving that it wasn’t special, the entire sky contained billions more galaxies than previously thought. The new estimate of the number of galaxies in the observable universe was 200 billion.
The term ‘‘observable universe’’ is important. It is the limit to the physical size of the Universe resulting from the fact that light has a finite speed.
The light from a source at the limit of the observable universe has just had time to reach us since the start of the Universe 13.8 billion years ago.
Light from sources beyond this limit hasn’t had time to reach us yet.
In 2009, the HST was serviced and upgraded instruments installed, after which the Ultra Deep Field observations were made. These revealed even fainter, more distant and more numerous galaxies.
In 2012 the results of detailed analysis of these Deep Field measurements were published by the HST team.
The experimental results and computer simulations concluded that the galaxy count made earlier was too small – by a factor of 10.
The accepted number of galaxies in the observable Universe now stands at 2 trillion – note that each of these galaxies contains on average 200 billion stars.
If each of the stars in the Universe has, on average one planet, this vast number of planets leads to speculation that the chances of some form of life existing elsewhere has to be good.
This isn’t the whole story since these figures relate only to the observable Universe – what lies beyond that, no one knows.