A new headcount of galaxies in the observable universe turned up 10- to 20 times more galaxies than previous estimates, bringing the tally up to as many as 2 trillion, a new study shows.
The missing members of the galactic family tree are most small, dim collections of stars that formed during the universe's early days. Space took up less space back then and the galaxies were crammed together, compared to today's expanded universe.
The Hubble Space Telescope peered long and deep into a small patch of the sky in the mid-1990s, revealing thousands of galaxies stretching back into time. Extrapolating from the analysis, scientists estimated the observable universe is filled with 100 billion- to 200 billion galaxies, said astronomer Christopher Conselice, with the United Kingdom's University of Nottingham.
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Now, a new analysis of the Hubble Deep-Field images, combined with other data, shows that estimate is just 10- to 20 percent of the total number of galaxies, most of which are too faint to be seen by present-day telescopes, Conselice and colleagues wrote in a paper to be published in The Astrophysical Journal.
The scientists converted Hubble and other telescopes' images into three-dimensional maps so they could calculate the density of galaxies and estimate the volumes of each successive region of space.
"This painstaking research enabled the team to establish how many galaxies we have missed - much like an intergalactic archaeological dig ... The results of this study are based on the measurements of the number of observed galaxies at different epochs - different instances in time - through the universe's history," the University of Nottingham wrote in a press release Conselice and colleagues found that there were significantly more galaxies at earlier times.
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"It appears that when the universe was only a few billion years old there were 10 times as many galaxies in a given volume of space as there are within a similar volume today," the scientists said.
"This is very surprising," Conselice added. "Over the 13.7 billion years of cosmic evolution since the Big Bang, galaxies have been growing through star formation and mergers with other galaxies. Finding more galaxies in the past implies that significant evolution must have occurred to reduce their number through extensive merging of systems."
"This gives us a verification of the so-called 'top-down' formation of structure in the universe," he added.
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