How Big Is The Universe?

NEW YORK, NEW YORK – APRIL 12: Professor Stephen Hawking onstage discussing his theories on the Creation of the Universe

Scientists such as Stephen Hawking have conjured up the theory of the Big Bang when a massive explosion occurred in Space and creation of the planets began. How can the universe be 95 billion light years across when it has only been in existence for approx 14.3 billion years? It;’s a real puzzler!

If nothing can travel faster than the speed of light shouldn’t the universe only measure approx 28.6 billion light-years across? The best estimate for the age of the universe from the Big Bang to now is 13.75 billion years give or take 0.11 billion years.

If the universe was static, you would theoretically be able to see 13.75 billion light years in any direction because that’s how far photons moving at the speed of light (about 300,000 kilometres per second in a vacuum) have travelled since shortly after the universe began. With Earth at the centre of the cosmos from our point of view, that would make the universe about 28.5-billion light-years wide.

But the universe isn’t static.

“[Since the Big Bang] the universe and space-time itself has been expanding,” says astronomer and astrophysicist Professor Paul Francis from the Australian National University’s Mount Stromlo Observatory .

“We know space is expanding it’s getting bigger and by extrapolating that backwards we can work out when everything we see was in the same place, and that gives us a rough age of the universe.” “The trouble is the universe may not have been expanding at the same rate, so extrapolating backwards may not be very accurate,” says Francis.

According to Francis, the universe is currently expanding at 70 kilometres per second per mega-parsec (1 parsec is 3.26 light-years or 31 trillion kilometres). So how long ago was everything together in the single place in space when the Big Bang happened?

“Since the time of the cosmic microwave background radiation 360,000 years after the Big Bang, the universe and everything in it has expanded away from everything else over 1100 times,” he says.

“These co-moving co-ordinates as they’re called, act like compound interest in finance, dramatically multiplying the size of the observable universe, giving it a diameter of about 93 billion light-years”. That puts the edge of the observable universe from Earth about 46.5 billion light-years away.

A universe wide radiation map of the big bang

Cosmic afterglow

To measure the expansion scientists look for tell-tale signs of cosmic ripples imprinted in the cosmic microwave background radiation — the afterglow from the Big Bang. These ripples measure 300 million light years across.

“Satellites such as W-Map and Planck can measure these ripples with a high degree of accuracy,” says Francis. “And we can also see them today because those originally very small ripples are now imprinted in the distribution of galaxies.”

“If you take a really big sample of galaxies, you’ll find they’re also in a ripple distribution and we know it’s the same scale as the cosmic microwave background radiation ripples.” “So we can measure what that scale is now and what it was when the universe was very young and that gives us a very accurate handle on how much the universe has expanded since then.”

“When combined with measurements of supernovae in nearby galaxies, it allows us to very accurately measure how the rate of expansion of the universe has changed over time, and this gives us a very accurate figure for the age of the universe.

“It’s like saying someone is 100 kilometres away from you travelling at 100 kilometres an hour, how long ago were they with you,” says Francis. Dr Paul Francis from the Australian National University’s Mount Stromlo Observatory was interviewed by Stuart Gary. Source: ABC Science

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