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August 8, 2023

The Science Perspective: Looking for Light with New Horizons

By Tod R Lauer

The galaxy stretches out in front of us. The solar system is behind us. We have traveled over 5 billion miles from the home we departed forever 17 years ago. We threaded our way through the moons of Jupiter, skimmed the frozen seas of Pluto, and buzzed Arrokoth, a fossilized remnant of the ancient solar nebula that created the Sun and the Earth. The space ahead is unknown.

We are now nearly twice as far away from the Sun as Pluto. The Sun has dwindled to a brilliant star, its illumination terribly diminished in power, compared to the wash of light that gives Earth life. We can still read by the Sun, but it gives no warmth, and no longer denies us the stars. The Milky Way is spectacular, stretching all the way around the sky. It’s not hard to see that we live in the plane of our galaxy. It’s still convenient to think of neat north and south hemispheres to organize our vista of the sky, but the equator is now defined by the band of the galaxy, not our spinning Earth, lost to us in the glare of the fading Sun.

But the Earth still speaks to us. Patience is required. A query must wait nearly 16 hours for an answer. The voice from Earth poses a long set of technical questions, carefully encoded in telemetry passed along by NASA’s Deep Space Network. The gist of it is simple, though. What do you see out there? Or maybe more precisely, what do you see that’s unknown to us?

The New Horizons spacecraft is still healthy, its cameras, spectrographs and sensors ready to respond. There is a bit of a trick involved, though. New Horizons was optimized to explore Pluto. It lacks the immense grasp of the Hubble or James Webb space telescopes. But those instruments huddle in Earth-space close to the blinding Sun – what might they be missing? The trick is improvisation. In flying our spacecraft across the solar system we’ve become skilled pilots. We can feel in our hands how a push here, a pull there shakes out a little performance that turns out to be just what was needed.

At the end of this summer, we will use our spacecraft to study the universe. It’s a proposition that at first seems ridiculous. Our main telescope is no more powerful than that any serious amateur astronomer is likely to have tucked away in their garage. We can only see bright nearby galaxies, and don’t have any instruments to diagnosis even their most basic properties. But it’s awfully dark where we are, and that gives us the power to do something that no one else can do. We can precisely measure how dark space itself is.

Credit: Serge Brunier/Marc Postman/Dan Durda

The James Webb Space Telescope was designed to see faint galaxies forming at the dawn of time. The Hubble Space Telescope has already spent three decades of sketching out the first rich picture of how galaxies once-formed evolved into the familiar objects we see around us today. From its observations, we conclude that there may be almost a trillion galaxies scattered around our night sky. And if the galaxies detected with Hubble, and now the Webb, give the whole story, then that’s that.

But is that really so? With our main camera, we can just simply ask how much light gets into it. We correct for all the light from galaxies that the Hubble says is there, that from the few stars that fall into view, and so on and so forth, and see if there is any light left over that’s unaccounted for. It’s an old idea that’s been tried many times. The trouble is that in Earth-space the Sun lights all the dust up in the inner solar system, and the glare is far, far brighter than anything to do with the universe beyond. There are any number of clever tricks to correct for it, but they have to work exactly right, and when all is said and done, there’s just too much uncertainly to say anything worthwhile. But from the vantage point of New Horizons in the outer solar system, we didn’t need any cleverness. We simply placed our camera far from the Sun.

We see something we don’t understand. We started by looking at old images taken for other purposes after we flew beyond Pluto. We had looked at a bunch of objects in the Kuiper Belt as we passed through it. All were faint and too far away to see any detail, but from our unique vantage point we could still learn a little about their formation. Many of them were far away from the bright band of the Milky Way, with the universe beyond as the only backdrop. The background levels in all cases were always brighter than could be accounted for by the billions of galaxies that we know are out there.

So, we then tried observations of a test field carefully selected to be far away from the Milky Way, any bright stars, clouds of dust – or anything, anything that we could think of that would wash out the fragile darkness of the universe. The total background was much lower than that in our repurposed images of Kuiper Belt objects – but by exactly the amount we expected, given our care at pointing the spacecraft at just about the darkest part of the sky we could find. The mysterious glow is still there, and more undeniable, given the care we took to exclude anything that would compete with the darkness of the universe, itself. You’re in an empty house, far out in the country, on a clear moonless night. You turn off all the lights everywhere, but it’s still not completely dark. The billions of galaxies beyond our Milky Way are still there, but what we measure is twice as bright as all the light they’ve put out over all time since the Big Bang. There is something unknown shining light into our camera. If it’s the universe, then it’s just as strong, just as bright, as all the galaxies that ever were.

Over the course of the next month or so, New Horizons is going to look at 15 more fields, likewise selected for exquisite darkness. We will wheel the spacecraft around the sky, keeping the Sun behind us, skirting the glowing Milky Way in front of us, looking between the galaxies, taking images of nothing in particular besides the face of the Universe, itself.

Fly across the solar system to the darkest skies anyone’s ever seen, and look for light - and in the light, knowledge about the universe.

Tod R. Lauer is an astronomer and New Horizons Science Team member from the National Optical-Infrared Astronomy Research Laboratory.