I was half way through reviewing this book but forgot to save and consequently here I am ...hours later starting afresh.The title promised much and when I flicked through the book before buying it looked very readable but authoritative. And I liked the way that if you only had an hour they suggested that you just read the large type, and if you had more time you could burrow down to the smaller fonts and even the diagrams. At the start it is a fairly straightforward review of the developments in cosmology: An earth centred universe; a sun centred universe; Friederich Bessel's measurement of parallax in 1838; realisation that there were a lot of stars and vast distances. Breakthrough with the Mt Wilson telescope in 1925 and discovery of Cepheid Stars which pulsate; Morley Michaelson ...speed of light same in all directions no matter if you are moving towards the source or away from source; Einstein and special relativity; Minkowski and the idea of time being another dimension (a four dimensional spacetime manifold); Einstein 1915 and General Relativity ...So this is how the universe can have shape...the matter in the universe can bend the lines of spacetime out of shape. (Demonstrated by gravitational lensing). But universe is not static but expanding ...how do we know? Answer: Red shift of light....no blue shift: so everything is moving away from us. Alexander Friedmann in 1922 showed that Einstein's field equations allowed for an expanding universe. Lemaitre predicted the further we look the faster things would appear to be moving...which is pretty much what Hubble found. But working backwards showed the Universe was 1-2 Billion years old. But that can't be right ..we knew the the earth was about 4 B years old. How come? In the 1930's it was realised that the Cepheid variables (which had been used for calibrating the red shift measurements) came in two forms; The true Cepheid variables and the intrinsically fainter but otherwise similar RR Lyrae stars. And using the latter had messed up Hubble's calculations. In 1948 Alder and Game demonstrated how matter and energy would have been interchangeable in the early universe ...in line with Einstein's equation E=mc squared so Big Bang theory began to be accepted and more so with the prediction of Cosmic Microwave Background Radiation (CMBR) and subsequent demonstration. So now an ultra fine CMBR map of the universe has been produced and the photons showing the red dots in it started on their journey to earth 13.7 Billion light years ago. However, as space has expanded since then at about the speed of light those red dots are "now" about 46 billion light years away.....and accelerating. Galaxies are receding by about 75 kilometres per sec per sec for every 3.26 million light years of distance. And for every 100 M light years we look into space we are effectively looking 100 M years back in time.
Questions about the Universe's shape always lead back to questions about mass. If we have a high mass density then we are likely to have a closed universe that will eventually collapse back into itself. If we have a low mass density then the universe will continue to expand at an accelerating rate, And if it's just right ...we'll have a flat universe...continuing to expand but not curving inwards or outwards. (Not sure that there is much practical difference between flat and expanding). Anyway, a whole lot of measurements and estimates have been made about density of stars and frequency and so estimates of the mass of the universe have been made....but Zwicky around 1933 found that galaxies were moving as though they contained 400X more mass than the starlight suggested. He called the "missing" mass dark matter and subsequent investigations showed it was universal. Best candidate today is for Weakly Interacting Massive Particles (WIMPs). Yet even with dark matter taken into account the omega factor (for open or closed universe) remained well below 1.....indicating an open universe. And measurements of anisotropy in ripples from CMBR (between normal matter and dark matter) indicated that omega was well below 1. So case closed: Open universe? Not quite. Measuring angular size of CMBR ripples indicate that the rays are parallel over the 46 billion light years ..so omega equals 1 and universe would be flat.....Ok but when you add normal (baryonic) matter and dark matter together it's not enough to give this result.
Enter dark energy. A type 1a supernova always releases the same amount of energy and therefore same peak luminosity...so can be used as a standard candle. But they are very rare. Nevertheless astronomers harvested data for 42 supernova with high red shifts. They expected that the very distant ones would be brighter than expected from a combination of red shift and the Hubble Constant ...in other words they would be starting to slow down. But Big surprise...the distant supernova were consistently fainter than predicted so cosmic expansion is accelerating. What's causing this? Explanation ..."Dark Energy". And measurements conclude that the mass/energy content of the universe is 5% ordinary matter, 27% dark matter and 68% dark energy. But at this level the universe still appears to be flat (Parallel lines really are parallel) whilst expanding at an accelerating rate.
If we could travel, faster than light. and get to the "edge" of our observable universe (currently 46 billion light years distant) we could look into regions forever hidden from earth based astronomers. And space would seem to be a vast sphere growing faster than the speed of light. Each position in the universe would be surrounded by its own sphere of observable distance (an infinity of universes). But this is just Level One Multiiverse of many such possible structures categorised by Max Tegmark. (Seems to me that these get increasingly dodgy and un-falsifiable). A Level 2 Multiverse can spin -off separate bubbles of Level 1 iMultiverses but these might have very different physical constants. The argument goes along the following lines: Our early universe apparently experienced a period of rapid inflation which allowed concentrations of mass. The best explanation for inflation was the energy released by the phase change of electro nuclear forces separating out. It's claimed the phases also apply to spacetime itself and the "vacuum energy" contained in spacetime itself can spontaneously give rise to new inflationary universes inside the old. This could be an explanation for the extra dimensions postulated to explain how the fundamental forces could be unified..eg in string theory. (Though recent commentary has been that string theory has gone nowhere and is essentially unfalsifiable so one can postulate anything). But other theories about unifying physics do not rely on the unproven notion of extra dimensions...eg Loop Quantum Gravity.
One way of resolving the quantum uncertainty problem (such as Schrodinger's cat...neither dead nor alive until observed) was the many worlds hypothesis of Hugh Everett. He suggested that the different outcomes of quantum events are resolved by the entire universe splitting into two divergent paths (Q: why only two?). So one universe where cat is alive, another where cat is dead. The structure of this Level 3 Multiverse would be like a branching tree ..giving rise to a fractal pattern. Obvious question is: where are all these other versions? The tricky answer is that they occupy exactly the same spacetime as we do. Most physicists interpret the many worlds proposal as a statement that our multiverse incorporates all the possible outcomes of quantum events within it. The wave function explaining this sort of multiverse can be described in terms of a Hilbert Space....A minority of physicists suggest that the universe really does branch to create new physical realities at every point in history. So a distinction is drawn between this position and the idea of quantum superpositions. And Tegmark suggested a Level 4 multiverse would be where a mathematical ensemble could give rise to all the other possible types of multiverse. This in turn gives rise to the idea that we could be in (part of) a mathematically simulated universe...but again...it's unscientific since it cannot be disproven).
Bottom line is: On current evidence, today's universe is "flat" extending essentially uniformly in all directions with no large scale curvature. But it almost certainly extends beyond the range of our visible universe into an effectively infinite Level 1 Multiverse. But because our universe has dark energy driving expansion it seems possible to have a universe that is both flat and likely to continue expanding forever. But another wrinkle in the scenario is that if dark energy is growing exponentially we might get the "big rip" in the cosmos where everything ..down to atoms are ripped apart by dark energy.
The question of why is the universe suspiciously fine tuned to foster the development of life: Two answers...... the weak anthropomorphic argument is that if it were not fine tuned we would not be here to observe it.......The strong Anthropic principle, described by Barrow and Tipler is that there is actually an imperative to give rise to life; either by a deity, or the universe could not exist without conscious entities to observe it (and resolve quantum outcomes)....Seems crazy to me....who was doing this before humans evolved after 4 billion years? Or, third variant..... there are multiverses such that there has to be an option somewhere like ours.
So some pretty weird ideas out there. But probably simplest to accept the idea of a flat universe currently accelerating under the influence of dark energy.
But if dark energy is populated totally on the basis of the distant type 1a supernovas being dimmer than expected...what if there are a sub type of 1a supernovas (just as there were two types of Cepheid type stars). Maybe the measurements are wrong. Seems to be a lot hanging on 42 type 1a Supernovas.
But a really good book. Delivers what it promises ...but wth a few deviations. ( )