The Hidden Reality talks about parallel universes and the deep laws of cosmos by Professor Brian Greene. It has been nominated for the Royal Society Winton Prize for Science Books for 2012.

Beautifully it gives a qualitative detail on the proposal of multiverse minus the mathematics behind each. The book illustrates the cutting edge thinking in theoretical physics about the notion of multiverse in the following nine different ways:

**Quilted Multiverse**: Universe here is infinite, configuration of matter repeat itself across space, thus, yielding parallel worlds.**Inflationary Multiverse**: It implies that Big Bang occurred due to force of anti-gravity that gave rise to network of bubble universes.**Brane Multiverse**: The existence of our observable universe is on one three-dimensional brane. Brane here refers to dynamical objects/giant membranes that can disseminate through spacetime according to the rules of quantum mechanics. And this three-dimensional brane is floating in a higher dimensional space with another unobservable three-dimensional brane. Something like bread loaves in a pack of bread, where each loaf is other parallel universe.**Cyclic Multiverse**: Collisions between brane-worlds give rise to Big Bang-like formation of the early universe. Thus, propagating universes that are parallel in time.**Landscape Multiverse**: It integrates inflationary cosmology and string theory thus yielding countless different bubble universes with various shapes and extra dimensions.**Quantum Multiverse**: Universes based on the randomness of quantum mechanics with a sort of observer bias or different possibilities in different junctions. We happen to be in just one branch that we occupy in the many-worlds interpretation of quantum mechanics.**Holographic Multiverse**: It is derived from the theory that the three-dimensional world where we live in is an illusion. While the ultimate precise reality is 2-dimensional reality at the surface of the universe.**Simulated Multiverse**: It implies that technological leaps suggest we develop enough computing power to create a simulation that has everything in the observable universe including sentient being. That world/simulated multiverse evolve further and create sufficient computing power to create more video games/simulation and so on.**Ultimate Multiverse**: It is based on the principle of fecundity, which states that “all possible worlds are real”. Thus, obviating the notion of ours is the special or the only reality.

## Inflation leading to Big Bang

Big Bang is the most accepted theory which is supposed to be responsible for our observable universe. In fact, our “time” starts at the Big Bang. However, what exactly led to Big Bang is still a mystery. Also, what was before that, is also questionable.

Now here comes the “inflationary theory”. The repulsive gravity that led to the “stretched quantum fluctuations”. As per the theory, when inflation ended, the field decomposed and morphed its energy into radiation and particles which are still around today.

## Many Bubbles Universes

Math behind the inflationary cosmology claims that the Big Bang like repulsion was not one time event. It may have been numerous times event. Thus, giving rise to many bubble universes, expanding in many cosmoses.

Universe began as primordial flash and that the galaxies were burning embers floating on the swelling sea of space.

## Ever-Expanding Universe

Georges Lemaître, Belgian Catholic priest, mathematician, astronomer, and professor of physics at the Catholic University of Louvain, recognized that in the math of Albert Einstein’s theory of general relativity is the idea of ever-expanding universe. Russian scientist Friedmann also had the similar idea.

Einstein, however, didn’t see it simply because he considered universe as static. In fact, for him, on large scale, universe was never dynamic. Data at that point in “time” suggested that universe is unchanging; after all, stars seem to be fixed in the night sky.

Now, Inflationary Multiverse comes into picture. Microwave background radiation or the heat left behind in the cosmos is an apt inference of expanding universe.

## Existence of Multiverse

The book postulates the possibility of an existence of multiverse. And that our observable universe is one amongst the many universes.

At the same time, it put forth the limitations. If there happens to be another universe, then we do not have access to it. Therefore, it’s difficult to prove.

Conventional physics works on following three ways:

- Generating theoretical ideas.
- The next obvious step is analysis, that is, testing the ideas
- If the ideas survive testing, physicists’ gain confidence in their theoretical ideas, else they throw them away.

This, however, cannot be done with multiverse. Currently, thinking to understand multiverse is not evolved much. Nevertheless, Greene is hopeful that with time, it might evolve. In principle though it’s possible.

Greene is of the view that if one understands the math behind multiverse, one can come upon a multiverse that may have favourable properties like we have it here.

## Cosmological Constant

Energy that pervades space has a value; we call it the cosmological constant. Quantum field jitters provide a microscopic mechanism that generates a cosmological constant.

The expansion of universe was discovered by scientists in 1990s. It turns out that the answer lies in Einstein’s equation of relativity that had a term cosmological constant. This, cosmological constant pushes the universe outwards. In a way, it’s a form of repulsive gravity. All this is part and parcel of Einstein’s mathematics.

But the problem is when one is to measure the amount of energy in space that would be responsible of observed speed up of cosmic expansion, the number is extremely small, that is, 10^{-123 }and this has been the biggest puzzle so far. Multiverse scenario explores an innovative way to find that number.

If there happens to be another universe, we can measure the repulsive gravity and with more universes we might get more values at hand. And somewhere in another universe, we may get very close to our existent cosmological constant.

Also, we can conclude that in universes where the cosmological constant to too high (with respect to us) the conditions will not be hospitable for our life forms to exist. With a higher repulsive gravity, galaxies don’t form, without galaxies there won’t be planets either.

## Why Multiverse?

Attempt to understand multiverse also helps us in following two ways:

- Why we see certain amount of dark energy? We are familiar with only 4% of universe because that’s our observable universe. And it is made up of
- protons
- electrons
- neutrons.

Therefore, we too are made up of same “stuff”.

We can tell what the other 96% of universe does, that is, in terms of gravity (it exerts) but we still have no insights as to:

- where it comes from and
- explain why it has particular value of mass component
- Certain percentage of universe is made up of dark matter / dark energy.

## String Theory and Extra Dimensions

Not only Professor Greene introduces us to a variety of multiverses theories, he also expounds his own area of research and the importance of String Theory. As per him, string theory is the key to understand the theories of multiverses.

**How?**

String theory is approach to realize Einstein’s dream of unified theory of physics.

A single theory to describe all the forces in universe.

Central idea of string theory is each and every matter is a filament of energy at the elemental level. Each vibrate in different patterns thus forming various particles such as electrons, quarks, neutrinos, photons etc.

There are internal inconsistencies in the theory, that is, math of string theory doesn’t work unless there is something unfamiliar and that is, the extra dimension space.

Putting this into the equation, string theory states, that at quantum level there are extra dimensions squeezed into a micro size that we have not yet detected.

Even though they are not visible to us, yet they can have huge influence on things that we can observe because shape of the extra dimension constrains how the strings can vibrate.

In string theory, vibration determines everything:

- Particle masses
- Strength of forces
- Also, amount of dark energy could be determined by the shape of the extra dimensions

## Shape of the Extra Dimension

Next obvious question would be, what is the shape of the extra dimension?

As per the mathematics, currently, we have 10^{500 }candidate shapes. Each with a different physical feature. Thus, implying at the possibility of multiverse.

It could be that each shape is a different universe with all together diverse atmosphere. Also, with a different amount of dark matter.

Therefore, regarding the above mentioned extremely small (10^{-123}) cosmological constant, Greene suggests that this amount of cosmological constant is congenial for life forms that we are aware of.

Thus, with string theory, universe can be explained in terms of tiny filaments that vibrate in 10-11 dimensions.

## Takeaway

I’m totally in awe with the book as it addresses some deep and big questions about reality. An interesting read to even those who have basic or no understanding of complex mathematical equations.