P.J.E. Peebles

3   Lessons

What are the lessons to be drawn from these examples of research in Windows on the Universe?

Here are my choices.

    1. We are seeking reality. Natural science operates on the hypothesis that nature operates by rules that can be discovered, in successive approximations, by the interplay of theory and observation. This is a social construction. There are far better examples than cosmology of how spectacularly productive the program has been, but still I am deeply impressed that in the decade since ΛCDM became predictive it has passed increasingly demanding tests of what has happened on length scales more than ten orders of magnitude larger than the Solar System.

    2. We won’t complete the search. I admire Weinberg’s book,¹⁰ Dreams of a Final Theory (by which is meant basic physics as opposed to its expression in complicated situations) but I suspect science will become final in the sense that we are unable to do better. Cosmology at redshift z > 10¹⁰ may end as a theory that is consistent internally and with all we know, adjustable to fit measurements as they come in, but not testable.

    3. We are still making progress, but it’s exceedingly uneven. A half century ago the search for measures of the large-scale structure of the universe seemed exceedingly ambitious, but technology has led to a convincingly established cosmology. The search for life in other worlds has a much longer history, but the window hasn’t opened yet. The physics of the dark sector of ΛCDM predicts that dark matter just piles up in halos while dark energy is constant or close to it. Physics in the visible sector is simple too, but capable of spectacularly complicated expression. Is the extreme simplicity of the dark sector only the easiest approximation we can get away with in a sector that has been only schematically explored? Returning to point 2, we may note that if ΛCDM physics differs from reality enough to matter it means it will be discovered, maybe in the laboratory, maybe in the astronomy.

    4. Our progress is socially-shaped. I risk reopening the science wars by offering a reminder of what I expect we’ve all experienced, that social forces help shape directions of research in the natural sciences. I mentioned examples in the search for life. I include examples in pure theory, as the anthropic argument. I include the influence of fashion, as in the relative attention paid to dark energy missions and our extragalactic neighborhood. This is an operating condition, and it can be productive, leading to motion toward point 1.

    5. Our results are durable but not to be trusted. Einstein wrote down general relativity theory nearly a century ago. His best test was the orbit of the planet Mercury. Now this theory passes demanding tests on the scale of the Hubble length, some 10¹⁵ times Mercury’s distance from the Sun. This is spectacular durability. But a half century ago thoughtful physicists, including Einstein, presented good arguments against the cosmological constant. Now we learn we almost certainly have to live with it. The dark sector of ΛCDM passes searching tests. It is properly used as the basis for large-scale numerical simulations of structure formation, and for designs of observational programs and analyses of the results. But the dark sector physics is not to be trusted; it certainly could be found to be more interesting than ΛCDM.

Taken from:

Lessons from Windows on the Universe