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There may be fewer galaxies in the universe than we thought

January 13, 2021. Summarized by summa-bot.

Compression ratio: 65.1%. 2 min read.

This photo shows a phenomenon known as zodiacal light. At lower left, a glowing patch extends to the upper right in the direction of Jupiter, the bright object left of center. Zodiacal light is caused by sunlight reflecting off tiny dust particles in the inner solar system???the disintegrated remains of comets and asteroids. Attempts to measure how dark space is using telescopes like Hubble have been thwarted by this ambient glow. As a result, astronomers relied on NASA???s distant New Horizons spacecraft to observe the sky free from zodiacal light. The faint background they measured is the equivalent of seeing a neighbor???s refrigerator light from a mile away. This very wide, multi-frame panorama was taken in October 2014 at Canyon de Chelly National Monument in northeast Arizona. The zodiacal light is at left, with the northern Milky Way to the right. The Orion constellation is at top right. Jupiter is the brighter object left of center, while a similarly bright object to the right (below Orion) is Sirius. M44 (the Praesepe Cluster) is just above Jupiter. On the horizon, a yellow glow marks the location of the nearby town of Chinle, Arizona.

A previous measurement by Hubble Space Telescope suggested there were 2 trillion galaxies spread across the universe. Now, the latest research points to only hundreds of billions of galaxies instead.

A previous measurement by Hubble Space Telescope suggested there were 2 trillion galaxies spread across the universe.

After NASA's New Horizons mission flew by Pluto and the distant object Arrokoth on the edge of our solar system 4 billion miles from Earth in 2015 and 2019, it looked across the vastness of black space.

The New Horizons spacecraft was at such a distance that the sky it surveyed was 10 times darker than the darkest sky observed by Hubble.

The previous estimate of galaxies was determined by astronomers counting every galaxy visible in Hubble's deep field and multiplying it based on the total area of the sky.

"Deep field observations are long-lasting observations of a particular region of the sky intended to reveal faint objects by collecting the light from them for an appropriately long time," according to the European Space Agency's Hubble site.

While space may appear absolutely black in its vast darkness, it's actually illuminated by the diffuse glow of distant stars and galaxies.

Leaving the illuminated inner solar system is the best way to determine just how many galaxies may exist in the unseen distance -- which is exactly what New Horizons did.

New Horizons found that distant galaxies are less plentiful than previously believed because the cosmic glow they cause is so weak.

"It's an important number to know -- how many galaxies are there?" said study coauthor Marc Postman, a distinguished astronomer at the Space Telescope Science Institute in Baltimore, in a statement.

"We simply don't see the light from two trillion galaxies. "

Previously, astronomers believed that 90% of the galaxies in the universe remained hidden from Hubble's view.

"Take all the galaxies Hubble can see, double that number, and that's what we see -- but nothing more," said lead study author Tod Lauer, an astronomer at the National Science Foundation's National Optical-Infrared Astronomy Research Laboratory, in a statement.

The background glow in the universe, called the cosmic optical background, is the visible light counterpart of the afterglow associated with the Big Bang, which is called the cosmic microwave background.

"It puts a constraint on the total number of galaxies that have been created, and where they might be in time. "

"New Horizons provided us with a vantage point to measure the cosmic optical background better than anyone has been able to do it," Lauer said.

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