Washington, April 24 : Astronomers have discovered large galaxies that have not changed much in size for the last 9 billion years, which suggests these cosmic giants were born 6-7 billion years earlier than expected.

The most popular model for galaxy formation is a bottom-up or hierarchical model in which small galaxies gradually develop into larger ones over a long period of time.

The model predicts that smaller galaxies would merge to form larger ones, growing to 90 percent of their size about 11 billion years after the Big Bang.

This means that smaller galaxies would be older and larger ones, younger.

A team of astrophysicists, led by Professor Chris Collins from Liverpool John Moores University (LJMU), used observations from the Subaru Telescope, as well as the XMM-Newton to collect data about galaxies that existed two-thirds of the way back in time since the Universe began.

By being able to extend the baseline of their data so far back in time, they have a foundation for concluding that large galaxies were almost fully formed 4-5 billion years after the Big Bang, 6-7 billion years earlier than the hierarchical model would forecast.

According to Dr. John Stott, who carried out the photometry and data analysis, "We were surprised to find that the largest and brightest galaxies in the universe have remained essentially unchanged for the last 9 billion years, having grown rapidly soon after the Big Bang."

The galaxies examined in the observations were brightest cluster galaxies (BCGs), located at the centers of galaxy clusters.

Because they are so bright and uniform, they are often used as beacons for studying the universe.

At the beginning of their research, the scientists examined the ages of the stars in the galaxies. Their findings indicated a consistent epoch of formation for the majority of the stars - about 2-3 billion years after the Big Bang.

Since the galaxies they studied were almost fully formed 4-5 billion years after the Big Bang, they concluded that they probably grew rapidly and formed much sooner than the hierarchical model of galaxy evolution suggests. (ANI)