Usually our telescopes point at an object we want to see in greater detail. In the 1990s, astronomers did the opposite. They pointed to the most powerful telescope in history, the Hubble Space Telescope, in a dark sky without known stars, gas or galaxies. But in that bit of nothing, Hubble revealed a breathtaking sight: the void was full of galaxies.
Astronomers have long wondered how many galaxies there are in the universe, but up until Hubble, the galaxies we could observe were far surpassed by fainter galaxies hidden by distance and time. The Hubble Deep Field series (scientists have made two more such observations) presented a kind of nuclear sample from the universe that returns almost to the big bang. This enabled astronomers to finally estimate the galactic population at least about 200 billion.
Why ‘at least’? Because even Hubble has its limits.
The further you go (and back in time), galaxies become harder to see. One cause of this is the pure distance that the light has to move. A second reason is the expansion of the universe. The wavelength of light from very distant objects is stretched (redshift) so that these objects can no longer be seen in the primary ultraviolet and visible parts of the spectrum that Hubble is designed to detect. Finally, the theory indicates that early galaxies were smaller and fainter to begin with and only joined together later to form the colossal structures we see today. Scientists are confident that these galaxies exist. We just do not know how many there are.
In 2016, a study was 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 much of those as yet unseen galaxies were hiding just outside Hubble’s eye. Added to existing Hubble observations, their results suggest that such galaxies make up 90 percent of the total, leading to a new estimate – that there may be up to two trillion galaxies in the universe.
However, such estimates are a moving target. As more observations roll in, scientists can play the variables better and increase the accuracy of their estimates.
This brings us to the most recent addition to the story.
After buzzing through Pluto and the bizarre Kuiper belt object, Arrokoth, NASA’s New Horizons spacecraft is on the edge of the solar system sailing to interstellar space – and recently it pulled a Hubble. In a study presented this week at the American Astronomical Society and soon in The Astrophysical Journal, a team led by astronomers Marc Postman and Tod Lauer, described what they found after training the New Horizons telescope on seven pieces of empty space to try to measure the level of ambient light in the universe.
According to them, they found that they could set an upper limit for the number of galaxies that exist and that the space is a little less than previously thought. According to their data, the total number of galaxies is more likely in the hundreds of billions, not trillions. “We simply do not see the light from two trillion galaxies,” Postman said in a release released earlier this week.
How did they come to their conclusion?
The search for perfect darkness
There’s another limitation on Hubble’s observations. Not only can it directly dissolve early galaxies, but it can also not detect their light due to the diffuse glow of ‘zodiacal light’. Due to a body of dust scattering light in the solar system, zodiac signs are very faint, but just like light pollution on earth, it can obscure even fainter objects in the early universe.
The New Horizons spacecraft has now escaped the realm of stellar light and is looking at the darkest sky ever imagined. It provides the opportunity to measure the backlight from outside our galaxy and compare it with known and expected sources.
Postman tells The New York Times that going further on an order of magnitude would not offer a darker view.
“If you have a telescope on New Horizons at the edge of the solar system, you can ask how dark the space is getting anyway,” Lauer wrote. “Use your camera only to measure the glow from the sky.”
Yet the measurement was not simple. In an article, astrophysicist and author Ethan Siegel, who did not participate in the study, explains how the team carefully identifies contributions from ‘camera noise, scattered sunlight, excess starlight from the axis, crystals of the spacecraft’s range’ , modeled and removed. and other instrumental effects. They also removed any images too close to the Milky Way. After all these things, they have left the slight glow of the universe, and that’s the exciting bit.
The 2016 study predicted that a universe with two trillion galaxies would produce about ten times more light than the galaxies we have observed so far. But the New Horizons team only found about twice as much light. This has led them to the conclusion that there are probably fewer total galaxies hiding there than previously thought – a number closer to the original Hubble estimate.
“Take all the galaxies that Hubble can see, double the number, and that’s what we see – but nothing more,” Lauer said.
Star Gazing: The Next Generation
These observations of New Horizons are not the end of the story. Our ability to see the earliest universe should give us a head start this year when Hubble’s successor, the James Webb Space Telescope (fingers crossed), starts and starts.
The JWST will observe at longer wavelengths than Hubble and is much larger. These properties should enable it to see the smaller, fainter galaxies even further. Like the Hubble Deep Field, if everything is in order, we can give an even clearer picture of the whole if we add the galaxies to the census.
Regardless of the number of scientists who end up there, it is unlikely to be extraordinarily large. Even a few hundred billion galaxies mean that there is a whole galaxy there for every star in the Milky Way. Such research will undoubtedly shed even more light on cosmological questions about how the universe formed. But it will also raise the question: are we really the only species that ever looks out and wonders if we are alone in the midst of the vast sea of galaxies, stars and planets?
Image Credit: eXtreme Deep Field / NASA