# The Observable Universe Just Got a Bit Smaller

## New Calculation Using Planck Satellite Data Updates the Size of Its Radius

By Guest Blogger Nick Tomasello, University of the Sciences in Philadelphia

The universe is an enormous place, so how much of it can we actually see? Not quite as much as we thought, it turns out. A recent study, conducted by Dr. Paul Halpern and myself using data from the Planck satellite, finds that the radius of the observable universe is actually slightly smaller than previously estimated. The findings, which update this radius from 45.66 billion light-years to 45.34 billion light-years (one light-year is 5.879 trillion miles), have been accepted for publication in the journal *Advances in Astrophysics*. A difference of 320 million light-years might be peanuts on the cosmic scale, but it does make our knowable universe a little bit cozier.

There are trillions of stars in the cosmos — way more than a mere human could possibly comprehend. So why is it that, no matter where on Earth we go, we can only see a few thousand (at best)? If, as was once assumed, the universe is both static and infinitely old, and light is the best way we have of observing things, there would have been enough time in the history of the universe for photons from every star in existence to have reached us. That would mean the night sky should be perpetually filled with bright light, which, thankfully, it obviously is not. This question, posed by a nineteenth century astronomer named Heinrich Olbers, marked one of the scientific community’s first steps in establishing why some of the universe is perceptible to us and the rest is far beyond our vision.

The blackness of the night sky is testament to the fact that the universe has a finite age. That age represents the maximum amount of time that light can have been traveling. Because photons move at a definite, finite speed, that naturally means there is a limit to the distance they can have covered over the lifespan of the universe. We call this limit the *particle horizon*, and it marks the bounds of the observable universe. Anything beyond the particle horizon is too far away for the photons it emits to reach us and thus can’t be seen even with the strongest telescope. It’s not that there isn’t anything in the dark patches between the night stars: any objects that are there are just beyond the reach of our vision.

The universe is huge, by the way, and it doesn’t have “bounds” and “edges” in the traditional sense. That means that the observable universe is subject to, well, the observer. We can think of the observable universe as a sphere or bubble centered on observers that follows them wherever they go. The limits of the sphere would be different for Earth, Mars, a star in the Andromeda galaxy, and so on.

Thinking of the observable universe like a bubble allows us to measure it in terms of radius. This radius, which is necessarily the same magnitude in all directions, serves as the distance from us to the particle horizon: it tells us the farthest we can see. But figuring out that distance is a little more complicated than just multiplying the speed of light by the age of the universe. Not only is the universe not infinitely old, it’s not static — it’s undergoing continuous expansion. It’s not that objects in the universe are moving apart due to their own power; on average, they actually stay at the same positions relative to each other. Instead, it’s more like the fabric of space-time is stretching, with the result that the distance between every point in the cosmos is increasing. You can picture this like the dots on an inflating basketball: as the ball gets bigger, the dots get further and further apart, although they retain their original orientation. To complicate matters more, in the late 1990s astronomers determined that this expansion is actually *accelerating* — the rate of growth between points is growing itself. Although we haven’t firmly determined why this is happening, it seems to be the result of a number of different factors, including inherent pressure from radiation and matter.

Why do these factors effect the bounds of our observable universe? As the fabric of space-time stretches, it tries to pull photons along with it, slowing their progress across the cosmos. To get a sense of this, imagine walking down the aisle of a train that’s moving in the opposite direction that you are: though you may be purposely moving forward, your overall progress isn’t as great because your surroundings are simultaneously moving you away. This means that, to accurately calculate the distance to the particle horizon, we have to account for a sort of “cosmic drag” on photons. To do this, astrophysicists measure something called the *scale factor*, which is a proportional quantity that keeps track of how much the universe has expanded. Because of acceleration, the scale factor increases with time. Astrophysicists include the effects of cosmic acceleration by estimating the pull of matter, radiation, and a certain undefined “dark energy” (unknown effect opposing gravity) often represented by a “cosmological constant.” Since all three of these might change dynamically throughout the eons, they too are time dependent.

Luckily, astrophysicists have developed precise techniques to help them measure these values and thus estimate the distance to the particle horizon. The first major calculation based on the accelerating universe was made by a team led by J. Richard Gott of Princeton in the early 2000s. Using data from the since-retired WMAP satellite, Gott’s team found that the radius of our observable universe was about 45.66 *billion *light-years. Gott’s calculation has served as a standard among the astrophysical community ever since, but, 10 years later, new cosmological data has called for an update.

Operating from 2009 to 2013, the Planck satellite scanned the surrounding expanse of space and provided more up-to-date figures about the expansion rate and other parameters of our universe. Those updates have nothing to do with how the universe changed in the intervening decade. While as time passes, the portion of the universe visible to us has slightly increased — as the universe ages, light from more and more distant objects has the time to reach us — that’s in a time-frame of billions of years and not significant to our measurements. Therefore, on the scale of just a decade, any change in parameters would just point to an increase in the accuracy of our data rather than reflect the dynamics of the universe itself.

And that’s exactly what happens: we applied the Planck data to a calculation of the radius of the particle horizon and found that it yielded a distance of about 45.34 billion light-years. Therefore, the range of what we can see is actually 0.7 percent smaller than once thought. In terms of the distances we’re dealing with it’s not a huge amount , but sometimes science has to take small steps.

We’ve come a long way since Olbers posed his paradox, as astronomy has become increasingly precise. Over the centuries we’ve discovered that the universe is far larger and more complex than we ever would have imagined on our own. It’s nice, then, when we find some data that shows us that at least one aspect of it, its radius of observation, is a little smaller.

Guest blogger Nick Tomasello recently received a B.S. in Physics from the University of the Sciences in Philadelphia and is currently enrolled in its Biomedical Writing program.

Reference: “Size of the Observable Universe,” Paul Halpern and Nick Tomasello, *Advances in Astrophysics*, 2016.