And: all the light. Or, as NASA sums it up, appropriately pragmatically and appropriately poetically: “the total amount of light from all of the stars that have ever shone.”
That light — photons from primordial stars, formed some 400 million years after the big bang — is still extant in the universe. It is more commonly known as extragalactic background light, or EBL — which is an accumulation of all the radiation in the universe …
… distant sources are blazars — compact quasars, or galactic nuclei — that boast more than a billion times the energy of visible light. Ajello and his team, for this project, studied 150 of them. Blazars are powered by massive black holes that emit jets of energy. And those jets include gamma rays. Gamma rays were the keys to the EBL project: When those rays collide with ancient photons, they’re converted into electrons and their antimatter (positrons). That collision effectively dims their light — meaning that gamma rays, when they finally hit our Fermi telescope, have an energy that belies their path through the universe. Using measures of that energy, Ajello and his colleagues were able to determine the amount of photons between Earth and the blazars. …
So, using this method, what’s the scientists’ best guess for the makeup of the universe’s ancient, photonic fog? The average stellar density in the cosmos, they estimate, is about 1.4 stars per 100 billion cubic light years. Which means that the average distance between stars in the universe is about 4,150 light years. …
There is plenty of space, some would say too much. The fastest human spacecraft so far travels at 213,200 mph. (See NASA just smashed the record for the fastest human-made object — its $1.5 billion solar probe is flying past the sun at up to 213,200 mph. – Nov 5, 2018) It will be slowed down, of course, as it leaves the solar system, but for the sake of argument, let’s use that as our top speed.
A light year is 5.88 trillion miles. (5.88 x 1012 mi), so the average distance (4,150 light years) to one star from another is about 24,402,000,000,000,000 miles. ( 243,963,00000000000 by a different calculator)
If I’m doing the math right it would take a rather disappointing average of over 13 million years for any one alien civilization to reach another, at current top human spacecraft speed. (13,065,743 years).
Do you have anything you’d like to say to some space aliens 13 million years in our future?
Even if aliens beat our top speed by 10 times, reaching 2.2 million miles per hour, the UFOs we see today could still have been traveling for over a million of years to reach us. (1,266,189 years at 2.2 million MPH to travel 4,150 light years.) Their original mission may have changed significantly since they left their home planet and they may look noting at all like their ancestors after so many generations.
They are also probably hungry and thirsty, so treat them well.