Re: How important is the UNobservable portion of the universe?

One of the great questions of all time is, "Where did it all begin?" That is, how old is the Universe, and, consequently, how big? Finding an answer to this question is difficult. However, with the realization that the Universe is expanding, Hubble provided a possible solution---just extrapolate backwards in time using the expansion rate of the Universe as a measure.

There have been many attempts to get an accurate value for the Hubble constant (which can be thought of as the "expansion rate of the Universe") and to measure the microwave background radiation that is the left over echo of the Big Bang. (Strictly one must measure the fluctuations in the cosmic microwave background.) Good measurements of either or both would provide some clues as to the age of the Universe.

Recently, NASA's Wilkinson Microwave Anisotropy Probe (WMAP) has been able to provide scientists with an unprecedented view of the Universe in the microwave region of the spectrum. In turn this has allowed for both the calculation of an accurate number for the Hubble constant and a measure of the background radiation that pins the age of the Universe as 13.7 billion years plus or minus 1%.

However, the WMAP also provided a detailed analysis of the composition of the Universe. It is 4% matter, 23% dark matter, and 73% dark energy. That is, it is 4% matter that we can see and 96% matter and energy that we can't. Indeed, we are not even sure what 96% of the Universe is composed of, let alone how to go about measuring it.

Put another way 96% of the Universe is "unobservable," at least with our current instruments, and I think that this is likely what your astronomer was referring to in the T.V. program. Moreover, because the speed of light is finite, that means that there is a fundamental limit to how far we can see. Naively one would expect this to be the speed of light times the age of the Universe. However, we have to take into account the fact that the Universe expands while light from distant regions propagates to us. The consequence is that the Universe would appear to be about 40 billion light years across. In a real sense, anything outside this region is unobservable because there has not been enough time for the light to propagate to us.

But as to the question of how relevant is the "UNobservable universe" to the size and age of the cosmos, the simple answer is "very"---if we are talking about dark stuff. The dark stuff appears to be pushing the Universe apart and aiding in the expansion of the cosmos. That is, at large distance scales, it is the opposite of gravity---driving galaxies further apart at an ever increasing pace. The dark stuff provides a lot of the overall control of the 4% that is matter and is critical to the Universe as we know it.

In the end, Einstein's biggest "blunder" - the cosmological constant that repels galaxies at a distance---may just show that he really did know what he was talking about. It would be interesting to find out what Einstein had to say about the dark stuff within the Universe.