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Census Finds Universe Has Ten Times More Galaxies
By David Reneke
The universe has ten times more galaxies than previously thought, and the Earth may have had more than one moon.
Astronomers using data from the NASA/ESA Hubble Space Telescope and other telescopes have performed an accurate census of the number of galaxies in the universe, and concluded that there are at least 10 times as many galaxies in the observable universe as previously thought. The results have clear implications for our understanding of galaxy formation, and also help solve an ancient astronomical paradox – why is the sky dark at night?
One of the most fundamental questions in astronomy is how many galaxies the universe contains. The Hubble Deep Field images, captured in the mid-1990s, gave the first real insight into this. Myriad faint galaxies were revealed, and it was estimated that the observable universe contains 100–200 billion galaxies.
Now, an international team led by Christopher Conselice of the University of Nottingham has painstakingly converted the Hubble images into 3D in order to make accurate measurements of the number of galaxies at different times in the universe’s history. In addition, they used new mathematical models that allowed them to infer the existence of galaxies that the current generation of telescopes cannot observe.
This led to the surprising realisation that in order for the numbers to add up, some 90% of the galaxies in the observable universe are actually too faint and far away to be seen right now.
In analysing the data, the team looked more than 13 billion years into the past. This showed them that galaxies are not evenly distributed throughout the universe’s history. It’s powerful evidence also that a significant evolution has taken place in which galaxies merged together, dramatically reducing their total number.
The decrease over time also contributes to the solution of Olbers’ paradox – why the sky is dark at night. The team concluded that every point in the sky contains part of a galaxy, but most are invisible to the human eye and even to modern telescopes.
A combination of factors like red-shifting of light and its absorption by intergalactic dust and gas all combine to ensure that the night sky remains mostly dark.
When Two Moons Collide
The Moon, and the question of how it was formed, has long been a source of fascination and wonder. Now a team of Israeli researchers have suggested that the Moon we see every night is not Earth’s first moon, but rather the last in a series of moons that orbited the Earth in the past.
The newly proposed theory runs counter to the commonly held “giant impact” scenario that the Moon is a single object that was formed following a single giant collision between a small Mars-like planet and the ancient Earth.
“Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth,” said Prof Hagai Perets of the Technion and Weizmann Institute in Israel. “It’s likely that such moonlets were later ejected, or collided with the Earth or with each other to form bigger moons.” To check the conditions for the formation of such mini-moons or moonlets, the researchers ran 800 simulations of impacts with the Earth.
The new model is consistent with science’s current understanding of the formation of the Earth. In its last stages of growth, Earth experienced many giant impacts with other bodies. Each of these impacts contributed more material to the proto-Earth until it reached its current size.
“We believe the Earth had many previous moons,” said Perets, who added that “a previously formed moon could therefore already exist when another moon-forming giant impact occurs”.
The tidal forces from the Earth could cause moons to slowly migrate outwards. The current Moon is slowly doing that at about 1 cm/year.
A pre-existing moon would slowly move out by the time another moon forms. However, their mutual gravitational attraction would eventually cause the moons to affect each other and change their orbits.
“It’s likely that small moons formed through the process could cross orbits, collide and merge,” said Perets. “A long series of such moon–moon collisions could gradually build up a bigger moon, which of course is the Moon we see today.”
David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com