Cosmology: Causal Horizons

My?June issue of Sky and Telescope magazine also had an interesting article on the creation and ultimate end of the Solar System. Though details are somewhat speculative, we know that the Sun is about halfway through its Main Sequence, so it will gradually get hotter, with unpleasant consequences for Earth. Toward the end of the main stellar sequence, the Sun will start expanding into a Red Giant, engulfing Mercury, and possibly Venus. Our now-blackened cinder planet will endure some billions of years of this; as the wreckage of Sol contracts into a white dwarf inside the death shroud of an expanding planetary nebula, what is left of everything will cool. Finally, orbitally destabilized Jupiter or Saturn, or possibly a wandering white dwarf, may, by gravitational slingshot, unceremoniously eject our dead Earth out into the deep void, far from its home solar system, wandering for trillions of years in uncharted space.

Mankind’s immediate concern, of course, is whether we will survive the next thousand years of living with each other – not what happens in the next 500 million years as we enter a terminally hot greenhouse phase of broiling jungle.

By 100 billion years or so, the article explains, Andromeda will have long since merged with the Milky Way, and the universe will have expanded so much that the sky, except for some stuff in our Local Group, will appear mostly void and black – those stars and galaxies having disappeared beyound our causal horizon.

Causal Horizon? That was a new one on me. I understand black-hole event horizons somewhat, where gravity is so intense not even light can escape. A causal horizon must be a horizon of a different color, and I knew there is a limit on how far out we can see, so I researched it on the web.

I found numerous references, both in the public domain (free) and in linked scientific papers that are not available to the general public without subscription. I would like to cite a brief passage and point you to the references below.

The NAS material on Cosmology is actually more extensive my referenced page link would indicate. Go to their full Cosmological Briefing if, after reading the excerpt, you want to read all the material – as I did.

The edge of the accessible universe is the causal horizon, a spherical boundary centered on Earth with a radius of about 15 billion light-years (the speed of light the age of the universe). Information from beyond the causal horizon cannot reach us because there has not been enough time since the Big Bang for any signal to travel so far, even at the speed of light. But as the universe gets older, the horizon moves out, bringing more of the unseen universe into view. What is the nature of the “stuff” beyond the horizon? Lacking information and an adequate physical theory of the Big Bang itself, cosmologists can only speculate.

As for the other, scientific papers found in my Google search: publication of scientific “publish or perish” papers has to be funded like any other endeavor. Indeed, even the full contents of the Astronomy magazine web edition are only available to paid subscribers (like myself). Yet I found Google listings of page after page of academic publications that are totally restricted, or, at best, synopsized in excerpts too brief to answer the thoughtful lay person’s question “what is the causal horizon”. What a shame. But, you see, not only did I find free resources, I found a second example of a causal horizon! The academic data is out there on the other side of the firewall, but we cannot see it.


Cosmology: A Research Briefing, National Academy of Sciences/National Research Council/National Academy Press

Closed Timelike Curve, Wikipedia

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