I’ve reminisced before about crossing Lake George in the Adirondacks in a small boat late one night some years back, when I saw the Milky with the greatest clarity I had ever experienced. Talk about dark skies! That view was not only breathtaking on its own, but it also raised the point about what we can see where. Ponder the cosmic optical background (COB), which sums up everything that has produced light over the history of the universe. The sum of light can be observed with even a small telescope, but the problem is to screen out local sources. No telescope is better placed to do just this than the Long Range Reconnaissance Imager (LORRI) aboard the New Horizons spacecraft.
Deep in the Kuiper Belt almost 60 AU from the Sun, the craft has a one-way light time of over eight hours (Voyager 1, by comparison, shows a one-way light time of almost 23 hours at 165 AU). It’s heartening that we’re continuing to keep the Voyagers alive even as the options slowly diminish, but New Horizons is still robust and returning data from numerous instruments. No telescope anywhere sees skies as dark as LORRI. That makes measurements of the COB as authoritative as anything we’re likely to get soon.
Image: Not my view from the Adirondacks but close. The Milky Way is gorgeous when unobscured by city lights. Credit: Derek Rowley.
The issue of background light came to the fore in 2021, when scientists at the National Science Foundation-funded NSF NOIRLab put data from New Horizons’ 20.8 cm telescope to work. That effort involved measuring the light found in a small group of images drawn from deep in the cosmos. It suggested a universe that was brighter than it should be, as if there were uncounted sources of light. Now we have further analysis of observations made with LORRI in 2023 supplemented by data from ESA’s Planck mission, which aids in calibrating the dust density in the chosen fields of view. We learn that contamination from the Milky Way can explain the anomaly.
The new paper from lead author Marc Postman (Space Telescope Science Institute) studies light from 16 different fields carefully chosen to minimize the background light of our own galaxy which, of course, surrounds us and compromises our view. This new work, rather than using archival data made for other purposes, explicitly uses LORRI to create images minimizing foreground light sources. The conclusion is evidently air-tight, as laid out by Postman:
At the outset of this work we posed the question: Is the COB intensity as expected from our census of faint galaxies, or does the Universe contain additional sources of light not yet recognized? With our present result, it appears that these diverse approaches are converging to a common answer. Galaxies are the greatly dominant and perhaps even complete source of the COB. There does remain some room for interesting qualifications and adjustments to this picture, but in broad outline it is the simplest explanation for what we see.
And let me throw in this bit from the conclusion of the paper because it adds an interesting dimension to the study:
If our present COB intensity is correct, however, it means that galaxy counts, VHE γ-ray extinction, and direct optical band measurements of the COB intensity have finally converged at an interesting level of precision. There is still room to adjust the galaxy counts slightly, or to allow for nondominant anomalous intensity sources.
In other words, to fully analyze the COB, the scientists have included VHE (very high energy) gamma ray extinction, meaning adjustments for the scattering of gamma rays as they travel to us. Although not visible at optical wavelengths, gamma rays can interact with the photons of the COB in ways that can be measured, as an adjustment to the rest of the COB data. That analysis complements the count of known galaxies and the optical band measurements to produce the conclusion now achieved.
I always find it interesting that there is both a deep satisfaction in solving a mystery and also a slight letdown, for let’s face it, odd things in the universe are fascinating, and let our imaginations run wild. In this case, however, the issue seems resolved.
I don’t have to mention to this audience how much good science continues to get done by having a fully functioning probe this deep in the Kuiper Belt. From New Horizons’ vantage point, there is little to no effect from zodiacal light, which is the result of sunlight scattering off interplanetary dust. The latter is a key factor in the brightness of the sky in the inner Solar System and has made previous attempts to measure the COB from the inner system challenging. We now look ahead to New Horizons’ search for other Kuiper Belt Objects to explore and try to learn whether there is a second belt of debris beyond the known one, and thus between it and the inner Oort Cloud.
We’ll doubtless continue to find things that challenge our assumptions as we press on, a reminder that a successor to New Horizons and the Voyagers is still a matter of debate both in terms of mission design and funding. As to the cosmic optical background, we give up the unlikely but highly interesting prospect that any significant levels of light come from sources unknown to us. As the paper concludes: “…the simplest hypothesis appears to provide the best explanation of what we see: the COB is the light from all the galaxies within our horizon..”
The paper is Postman et al., “New Synoptic Observations of the Cosmic Optical background with New Horizons,” The Astrophysical Journal Vol. 972, No. 1 (28 August 2024), 95 (full text). The 2021 paper is Lauer et al., “New Horizons Observations of the Cosmic Optical Background,” The Astrophysical Journal Vol. 906, No. 2 (11 January 2021), 77 (full text).
Universe is a word that gets much bandied about and gets confused with horizon. To the writers credit, the distinction is clearly made.
From a practical perspective, the light horizon is the knowable universe. We can never receive information in any form from beyond it. Yes our horizon is expanding, but at a slow, relentless pace.
From a philosophical perspective, ‘universe’ may be a foam of expanding horizons around countless big-bangs in a beginningless, eternally-expanding spacetime. In time the horizons evaporate and disappear over trillions of years into heat-death within the general expansion. Only the universe is eternal.
An interesting possibility is the intersection of light horizons from different big-bang events. This would explain the discovery of galaxies that seem to be morphologically much older than the generally agreed time elapsed since our own big-bang event. If a directional component to the COB could be discerned this would be amazing.
Why is the sky dark at night?
“Olbers’ paradox, in cosmology, paradox relating to the problem of why the sky is dark at night. If the universe is endless and uniformly populated with luminous stars, then every line of sight must eventually terminate at the surface of a star. Hence, contrary to observation, this argument implies that the night sky should everywhere be bright, with no dark spaces between the stars. This paradox was discussed in 1823 by the German astronomer Heinrich Wilhelm Olbers, and its discovery is widely attributed to him. The problem was considered by earlier investigators and can be traced back to Johannes Kepler, who, in 1610, advanced it as an argument against the notion of a limitless universe containing an infinite number of stars. Various resolutions have been proposed at different times. If the assumptions are accepted, then the simplest resolution is that the average luminous lifetime of stars is far too short for light to have yet reached the Earth from very distant stars. In the context of an expanding universe, it can be argued similarly: the universe is too young for light to have reached the Earth from very distant regions.”
https://www.britannica.com/science/Olbers-paradox
That isn’t correct, is it? We have light from galaxies just a few hundred my after the formation of our universe with the “Big Bang”. There are other effects, but the light not reaching us is wrong. If stars are not luminous long enough can be modeled. The luminosity per distance shell should decline with modeled extinction rates.
Olber’s Paradox has been around for a long time (I remember reading about it as a kid, probably in George Gamow’s “One, Two, Three…Infinty!”). Its nice to see it is still getting people to asking embarrassing questions.
Here is a more informative reference:
https://en.wikipedia.org/wiki/Olbers%27s_paradox
Hang on, the article says, to explain their observations, the number of small galaxies has been underestimated by a factor of 2 ?
Olber’s paradox requires a static universe, though the Big Bang does present something similar. At one point the entire universe was as bright as the surface of a star. As the universe expands, this light has been red shifted well out of the band of visible light into the CMB.
Here is a complex article behind an trivial appearance. First comment: who has never done astrophotography even with amateur devices, can not imagine the difficulty of taking a picture of the night sky and capture the light of a distant object: dust, thermal effects, light pollution, vibrations; transformation of the signal on the CDD sensor (the article talks about it) etc It takes a formidable precision to LORRI which is a marvel of technology.
https://pluto.jhuapl.edu/Mission/Spacecraft/Payload.php
The challenge is all the more difficult as it is not a matter of measuring ONE object but to work on a vast angular field bathed in light even at the galactic poles. The technical protocol described in the article seems to me perfectly useful for obtaining a most precise measurement. There is however a naive question that I would like to ask: can the intrinsic amount of energy of a photon vary independently of the effects of occultation by the galactic dust? If we assume that the photon loses or gains energy when it is in the LORRI axis, the light variation perceived on the CDD will also vary. Is this possible? Once released from the initial plasma, all the photons of the COB have keeped the same energy?
My second comment is philosophical : our universal horizon has been expanding for centuries depending on the quality of our instruments. We see further because our technology is more precise but at the same time, these “visual” limits of our universe seem to move diabolically backwards every time, as if to taunt us: that’s all the beauty of it. So we can ask ourselves the question of how far our technology will allow us to “look” at the universe?
it’s a bit off topic but I suggest here this excellent book by Miller which retraces the whole history of “chipset”, very interesting to understand the challenges of over-miniaturization.
https://www.amazon.fr/Chip-War-Worlds-Critical-Technology/dp/1982172002
The word “horizon” is also interesting. Greek ethymology means: to set boundaries. The human species needs limits to understand its universe, limits that it constantly pushes, as if our exploration were done in a sequential way. Apparently, it was impossible for us to consider a totality of things (unlike our friends of type III) I like the name “New Horizon” it sounds a bit like the trolleys of the conquest of the west in the 19th century…bigger and more expensive:)
See this great engraving of the Middle Ages where man was represented by the celestial vault:
-> https://ibb.co/r4pR8SF
From a SETI point of view, if we could completely filter the known light sources of the COB, could we detect a luminous technosignature ETI either because it would become visible or by occultation? see here this surprising photo taken by LORRI
https://photojournal.jpl.nasa.gov/instrument/LORRI?subselect=Target%3AKuiper+Belt+Object%3A
Yes, the finite age of the Universe is the primary factor in explaining Olber’s Paradox.
See
https://articles.adsabs.harvard.edu/pdf/1991ApJ…367..399W
https://ui.adsabs.harvard.edu/abs/1991ApJ…367..399W/abstract
The notion of ‘filtering’ the COB is intriguing.
But how is the image you referenced ‘surprising’? Perhaps I’m missing your point; how is this example different from other occultations?
@mike
Sorry I misspoke: imagine that we filter completely and perfectly the images of the LORRI we should see a globally coherent COB. if a light source is more intense – galaxy or ETI? – it would appear then.
I see it now. What an intriguing idea.