How big is the universe?Posted: January 7, 2015
Devoted ten minutes to the puzzle, after discovering I myself had no idea really.
Let’s start with basics.
One problem is it’s hard to render these distances on any map. Take just sun to Earth, for instance.
Sun to Earth
93 million miles.
The earth’s diameter is 7,918 miles. The sun’s diameter is 864,327 miles. So if we made a map, where the Earth was one inch, the sun would have to be nine feet tall and 978 feet away.
Another way: if Earth is a golf ball (1.68 inches diameter) the sun is a 15.26′ ball, five and half football fields away.
OK, how about to the edge of our solar system?
Well, what’s the edge? Neptune’s the most distant planet, right (after that unpleasant Pluto business)?
Sun to Neptune
4.18 light hours (or .00047684 light years)
But the real edge of our solar system, people seem to think, is way crazy farther past even Pluto.
It’s in a place that is still mostly just a theory, a sphere of wandering ice comets called the Oort Cloud:
Very difficult to render how far away the Oort Cloud is, at this level the scaling is so ridiculous that a 2D map with like dots on it becomes pretty irrelevant.
From the sun to the Oort Cloud – the edge of the solar system – is something like 1.87 light years.
The next solar system over, Alpha Centauri, is 4.37 light years from the sun.
VERY good chart, thanks so much NASA/Penn and also for putting that map in the public domain, although I guess as a federal taxpayer I do own it kind of.
You can see Alpha Centauri with your naked eye, I believe, I think our excellent friend Jeff even pointed it out to us once. From Earth it appears to the eye as a single object even though it’s a two-star system:
Us and Alpha Centauri are in the Milky Way. You can see the Milky Way from Earth, even though we’re in it, because it’s a spiral, and we’re in the spiral:
The laser in this picture is pointing toward the Galactic Center, which is 27,000 light-years away from the sun.
The Milky Way is 100,000-120,000 light years in diameter.
How many stars are in there? Maybe: 100-400 million stars they think. These numbers are much revised over history and expect will be revised many times again.
The closest galaxy over is Andromeda, which is 2.5 million light-years from Earth.
It is in our (comically named) “Local Group,” which has more than 54 galaxies in it.
Comically named I say because the diameter of the Local Group is 10 million lightyears.
From Earth to the edge observable universe in any direction is 46 billion lightyears.
What’s that you say? How can that be? Back up.
Does the universe have a center? Are we the center?
If we’re the center then… what? If not then… what?
Well at this point, I’m afraid I’ve lost comprehension for now and more reading would be necessary to even begin to wrap a desperate brain-finger around the most basic essays into this fathomless question.
Many secondary sources have reported a wide variety of incorrect figures for the size of the visible universe. Some of these figures are listed below, with brief descriptions of possible reasons for misconceptions about them.
13.8 billion light-years
The age of the universe is estimated to be 13.8 billion years. While it is commonly understood that nothing can accelerate to velocities equal to or greater than that of light, it is a common misconception that the radius of the observable universe must therefore amount to only 13.8 billion light-years. This reasoning would only make sense if the flat, static Minkowski spacetimeconception under special relativity were correct. In the real universe, spacetime is curved in a way that corresponds to the expansion of space, as evidenced by Hubble’s law. Distances obtained as the speed of light multiplied by a cosmological time interval have no direct physical significance.
- 15.8 billion light-years
- This is obtained in the same way as the 13.8 billion light year figure, but starting from an incorrect age of the universe that the popular press reported in mid-2006. For an analysis of this claim and the paper that prompted it, see the following reference at the end of this article.
- 27.6 billion light-years
- This is a diameter obtained from the (incorrect) radius of 13.8 billion light-years.
- 78 billion light-years
- In 2003, Cornish et al. found this lower bound for the diameter of the whole universe (not just the observable part), if we postulate that the universe is finite in size due to its having a nontrivial topology, with this lower bound based on the estimated current distance between points that we can see on opposite sides of the cosmic microwave background radiation(CMBR). If the whole universe is smaller than this sphere, then light has had time to circumnavigate it since the big bang, producing multiple images of distant points in the CMBR, which would show up as patterns of repeating circles. Cornish et al. looked for such an effect at scales of up to 24 gigaparsecs (78 Gly or 7.4×1026 m) and failed to find it, and suggested that if they could extend their search to all possible orientations, they would then “be able to exclude the possibility that we live in a universe smaller than 24 Gpc in diameter”. The authors also estimated that with “lower noise and higher resolution CMB maps (from WMAP’s extended mission and from Planck), we will be able to search for smaller circles and extend the limit to ~28 Gpc.” This estimate of the maximum lower bound that can be established by future observations corresponds to a radius of 14 gigaparsecs, or around 46 billion light years, about the same as the figure for the radius of the visible universe (whose radius is defined by the CMBR sphere) given in the opening section. A 2012 preprint by most of the same authors as the Cornish et al. paper has extended the current lower bound to a diameter of 98.5% the diameter of the CMBR sphere, or about 26 Gpc.
- 156 billion light-years
- This figure was obtained by doubling 78 billion light-years on the assumption that it is a radius. Since 78 billion light-years is already a diameter (the original paper by Cornish et al. says, “By extending the search to all possible orientations, we will be able to exclude the possibility that we live in a universe smaller than 24 Gpc in diameter,” and 24 Gpc is 78 billion light years), the doubled figure is incorrect. This figure was very widely reported. A press release from Montana State University – Bozeman, where Cornish works as an astrophysicist, noted the error when discussing a story that had appeared in Discover magazine, saying “Discover mistakenly reported that the universe was 156 billion light-years wide, thinking that 78 billion was the radius of the universe instead of its diameter.”
180 billion light-years
This estimate combines the erroneous 156 billion light-year figure with evidence that the M33 Galaxy is actually fifteen percent farther away than previous estimates and that, therefore, the Hubble constant is fifteen percent smaller. The 180 billion figure is obtained by adding 15% to 156 billion light years.
OK, friend, you lost me. You’re on your own.
I guess the point is whether or not I do, today, finally remember to buy paper towels is not super important.