Yesterday I wrote about the inner space within the atom, of the force holding the quarks that form the neutron together, the nature of reality at the subatomic level, billions of billions times smaller than anything our eyes can see. The fundamental building blocks of our universe.
Yesterday I was juggling extremely small numbers, such as the diameter of a hydrogen atom. And something ten billion times smaller than that - the smallest distance that's theoretically possible - the Planck length (1.616 x 10-35 meters).
Today, we will be zooming right out to the furthest fringes of the observable Universe. Today I write about outer space - our solar system, our galaxy, and the 200 billion or so galaxies that make up what we currently perceive as the observable universe.
Since last year's Lent, the James Webb Space telescope has been successfully deployed. It has been transmitting stunning images of distant galaxies, as well as discovering exoplanets and trawling through our own asteroid belt. It is expected that it will deliver as many cosmological discoveries as the Hubble space telescope did before it, expanding the size of our observable universe.
One thing I learnt this past year from the numerous astronomy talks I've watched on YouTube is that the Oort Cloud, a shell of icy particles lying beyond furthest fringes of our solar system, extends out to some 3.2 light years from the Sun. This is about three quarters of the way between us and the nearest star system to ours - Proxima Centauri. Now, Proxima Centauri is a red dwarf star, one-eighth of the mass of our sun. If it has its own equivalent of an Oort Cloud, this should extend out to less than half a light year from Proxima Centauri. Near neighbour on our doorstep, in other words.
Proxima Centauri is located about four and a quarter light years from our sun. Our galaxy, the Milky Way, is 87,400 light years across, and contains up to 400 billion stars. The nearest galaxy to the Milky Way is Andromeda, 2.5 million light years away. We are seeing it today as it looked during the Early Pleistocene epoch on earth, before the emergence of the earliest hominids. Andromeda contains around a trillion stars. The Milky Way and Andromeda are just two galaxies in the Virgo Supercluster, our local supercluster, which contains around 100 galaxies. It's 110 million light years across.
The observable Universe stretches 41.5 billion light years in each direction from us and contains around 200 billion galaxies (each of around 100 million stars on average). Beyond the edge of the observable Universe? We just don't know; light from beyond that distance isn't observable. It could be bigger - much vaster, but finite - or else it could be infinite - or it could be one of a number of universes, an infinite number even. We don't know.
However, we do know that just under 5% of our observable Universe is made up of matter - the atoms I wrote about yesterday, protons and neutrons, each made up of three quarks, bound together by the strong atomic force in a nucleus, surrounded by an electron shell [remember, hydrogen has no neutron]. We can postulate that the rest of the matter and energy of the observable Universe consists of dark matter (holding galaxies together) and dark energy (pushing galaxies away from one another). Although no one has detected either dark matter or dark energy, they are calculated to make up around 27% (dark matter) and 63% (dark energy) of the contents of the Universe - the 95% that isn't ordinary matter.
We know that Big Bang happened 13.8 billion years ago. So why is the observable universe more than 13.8 billion light years in radius? It suggests that spacetime has expanded three times faster than light...
Trying to work out how fast the Universe is expanding isn't easy. The Hubble Constant, used to determine the speed with which galaxies are flying apart, has been variously calculated as being between 63 and 73 kilometres per second per megaparsec, is proving hard to hammer down into a number which squares with all observations. Again, right now, we don't know.
The reality of the observable Universe remains unknown to science ('the crisis in cosmology'), yet it's the same science that works so well on our human scale, that brings us such civilisational advances as the laptop and internet on which I am currently writing.
To help me put the subatomic and the galactic into perspective, I asked ChatGPT how many cubes, with each side one Planck length long, would fit into the observable Universe. Here's its answer:
The Planck length is approximately 1.616 x 10-35 meters, and is believed to be the smallest measurable length scale in the universe.
To calculate the number of cubes with each side being a Planck length that can fit into the volume of the observable universe, divide the volume of the observable universe by the volume of one cube. The volume of one cube with each side being a Planck length is (1.616 x 10-35)3 = 4.224 x 10-105 cubic meters.
Dividing the volume of the observable universe (4 x 1080 cubic Planck lengths) by the volume of one cube (4.224 x 10-105 cubic meters) gives: (4 x 1080) / (4.224 x 10-105) = 9.47 x 10184 cubes.
Therefore, you can fit approximately 9.47 x 10184 cubes, with each side being a Planck length, into the volume of the observable universe*.
That's the biggest meaningful number in our Universe - of course you could continue multiplying it by successive orders of magnitude - but so what?
***** ***
The reason that I have been in recent years fascinated by physics and cosmology boils down to my quest for a better understanding of God.
If we are to reconcile science and spirituality, we need to start with an understanding of the physical nature of our reality, at the smallest and largest levels imaginable. And then to look for purpose - why all this came into being - and why our consciousness is here to observe it?
* UPDATE 2024: I asked Google Gemini the same question. It came up with exactly the same order of magnitude, but preceded by the number 5.44 rather than 9.47, and with a more subtle answer:
In reality, packing identical spheres (analogous to cubes here) efficiently into a larger space leads to wasted space. The best known packing efficiency for spheres is around 74%, achieved by the Kepler conjecture. Applying this factor reduces the number of cubes by 26%, resulting in about 3.96 x 10184 cubes.
Lent 2022: Day three
Gratitude and Consciousness
Lent 2021: Day three
Would the Universe exist without us?
Lent 2020: Day three
Define your Deity
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