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Usually we point the telescopes at an object that we want to see in more detail. In the 1990s, astronomers did the opposite. They pointed the most powerful telescope in history, the Hubble Space Telescope, at a dark area of the sky devoid of known stars, gas, or galaxies. But in that burst of nothingness, Hubble revealed a breathtaking spectacle: the void was teeming with galaxies.
Astronomers have long wondered how many galaxies there are in the universe, but up to Hubble, the galaxies we could observe were far outnumbered by weaker galaxies hidden by distance and time. The Hubble Deep Field series (scientists made two more such observations) offered a sort of basic sample of the universe dating back almost to the Big Bang. This allowed astronomers to finally estimate the galactic population to be at least about 200 billion.
Why “at least”? Because even Hubble has its limits.
The further you go (and go back in time), the galaxies become harder to see. One of the causes is the sheer distance that light has to travel. A second reason is due to the expansion of the universe. The wavelength of light from very distant objects is stretched (shifted red), so these objects can no longer be seen in the predominantly ultraviolet and visible parts of the spectrum that Hubble was designed to detect. Finally, the theory suggests that the early galaxies were smaller and weaker to begin with and only merged later to form the colossal structures we see today. Scientists are convinced that these galaxies exist. We just don’t know how many there are.
In 2016, a study published in The astrophysical journal by a team led by Christopher Conselice of the University of Nottingham, used a mathematical model of the early universe to estimate how many of these still invisible galaxies are lurking just beyond Hubble’s view. Added to existing Hubble observations, their results suggest these galaxies make up 90% of the total, leading to a new estimate – that there may be as many as two. thousand billion galaxies in the universe.
Such estimates, however, are a moving target. As new observations arrive, scientists can better understand the variables involved and increase the precision of their estimates.
Which brings us to the most recent addition to the story.
After buzzing by Pluto and the bizarre Kuiper Belt object Arrokoth, NASA’s New Horizons spacecraft is at the edge of the solar system cruising into interstellar space – and recently it fired a Hubble. In a study presented this week to the American Astronomical Society and soon to be published in The astrophysical journal, a team led by astronomers Marc Postman and Tod Lauer described what they found after training the New Horizons Telescope over seven shards of empty space to try and measure the level of ambient light in the universe.
Their findings, they say, allowed them to set an upper limit on the number of galaxies in existence and indicate that space may be a little less crowded than previously thought. According to their data, the total number of galaxies is more likely in the hundreds of billions, not in the trillions. “We just don’t see the light of two trillion galaxies,” Postman said in a statement earlier in the week.
How did they come to their conclusion?
The search for perfect darkness
There is another constraint on Hubble’s observations. Not only can it not directly resolve the first galaxies, but it cannot even detect their light due to the diffuse glow of “zodiacal light”. Caused by a dust halo scattering light in the solar system, zodiacal light is extremely weak, but just like light pollution on Earth, it can obscure even paler objects in the early universe.
The New Horizons spacecraft has now escaped the realm of zodiacal light and is gazing at the darkest sky yet. This provides the ability to measure background light beyond our galaxy and compare it to known and expected sources.
The postman said The New York Times going an order of magnitude further would not have offered a darker view.
“When you have a telescope on New Horizons, at the edge of the solar system, you may wonder how dark space is getting anyway,” Lauer wrote. “Use your camera only to measure sky glow.”
Yet the measure was not straightforward. In an article, astrophysicist and writer Ethan Siegel, who was not part of the study, explains how the team meticulously identified, modeled, and removed the contributions of “camera noise, scattered sunlight, off-axis excess starlight, crystals from the spacecraft thrust, and other instrumental effects. They also removed any images that were too close to the Milky Way. After all of that, they ended up with the faint glow of the universe, and that’s what’s exciting.
The 2016 study predicted that a universe with two trillion galaxies would produce about ten times more light than the galaxies we’ve seen so far indicate. But the New Horizons team found only about twice as much light. This led them to the conclusion that there are probably fewer total hidden galaxies than previously thought – a number closer to Hubble’s original estimate.
“Take all the galaxies Hubble can see, double that number, and that’s what we see – but nothing more,” Lauer said.
Star Gazing: the new generation
These New Horizons sightings are not the end of the story. Our ability to see the oldest universe should take a head start this year when (fingers crossed) Hubble’s successor, the James Webb Space Telescope, launches and begins operations.
The JWST is configured to observe at longer wavelengths than Hubble and is much larger. These attributes should allow him to see even further back and imagine those first smaller and weaker galaxies. Like the Hubble Deep Field, if everything is in working order, adding these galaxies to the census should give us an even clearer picture of the whole.
Whatever number of scientists eventually land on, it’s unlikely to be anything but huge. Even a few hundred billion galaxies mean that there is a all galaxy there for every star in the Milky Way. Such research will undoubtedly shed even more light on the cosmological questions about the formation of the universe. But it will also raise the question: in the midst of the vast sea of galaxies, stars and planets, are we really the only species to look and wonder if we are alone?
Image Credit: eXtreme Deep Field / NASA
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