On Friday at UQ we had a very stimulating colloquium A new view on quantum gravity and the origin of the Universe by Bei-Lok Hu (University of Maryland). A key aspect of this new view is that general relativity and space time should be viewed as emergent phenomena (more below).

There are six main points of experimental evidence in cosmology:

1. Hubble expansion of the universe.

2. Cosmic microwave background radiation (isotropy and uniformity).

3. Element abundance (+ nucleosynthesis)

4. Ratio of baryon/photon (entropy content of universe)

5. Structure: galaxy, clusters,... hierarchy of scales

6. Cosmological constant ~ 0, vacuum energy density

The fact that the night sky is dark implies a finite universe, and expansion or a hierarichial structure (Olber's paradox).

Hu contrasted Two views of quantum gravity.

1. Bottom up view

Quantum gravity = quantisation of general relativity

This is the more traditional view and has been dominant.

2. Top down view

Gravity is emergent and general relativity should be viewed as the "hydrodynamics" of some underlying "microscopic" theory.

This means that one must deal with a micro-macro transition as well as a quantum-classical one. This view has become more popular in the last 5 years.

Hu's advocacy of 2. is summarised in detail in a conference paper.

It should be pointed out that Bob Laughlin has also advocated such a perspective. His book,

[See also his 2004 Perspective, The Cup of the Hand, in Science].

On monday we will have another colloquium, this one by Thanu Padmanabhan (IUCAA, Pune University), and advocating a similar view, and summarised in this conference paper.

There are six main points of experimental evidence in cosmology:

1. Hubble expansion of the universe.

2. Cosmic microwave background radiation (isotropy and uniformity).

3. Element abundance (+ nucleosynthesis)

4. Ratio of baryon/photon (entropy content of universe)

5. Structure: galaxy, clusters,... hierarchy of scales

6. Cosmological constant ~ 0, vacuum energy density

The fact that the night sky is dark implies a finite universe, and expansion or a hierarichial structure (Olber's paradox).

Hu contrasted Two views of quantum gravity.

1. Bottom up view

Quantum gravity = quantisation of general relativity

This is the more traditional view and has been dominant.

2. Top down view

Gravity is emergent and general relativity should be viewed as the "hydrodynamics" of some underlying "microscopic" theory.

This means that one must deal with a micro-macro transition as well as a quantum-classical one. This view has become more popular in the last 5 years.

Hu's advocacy of 2. is summarised in detail in a conference paper.

It should be pointed out that Bob Laughlin has also advocated such a perspective. His book,

*A Different Universe*, has a chapter,*The Fabric of Space-Time*, which ends with the claim that if Einstein were alive today he would,conclude that his beloved principle of relativity was not fundamental at all but emergent - a collective property of the matter constituting space-time that becomes increasingly exact at long length scales but fails at short ones. This is a different idea from his original one but something fully compatible with it logically, and even more exciting and potentially important. It would mean that the fabric of space-time was not simply the stage on which life played out but an organizational phenomenon, and that there might be something beyond.R.B. Laughlin,

*A Different Universe*, p. 126[See also his 2004 Perspective, The Cup of the Hand, in Science].

On monday we will have another colloquium, this one by Thanu Padmanabhan (IUCAA, Pune University), and advocating a similar view, and summarised in this conference paper.

The orthodox view is that these ideas don't work because of the "Weinberg-Witten theorem", a no-go theorem which says you can't make a massless spin 2 boson out of gauge bosons, and that string theory gets around this because the theorem implicitly assumes that the graviton will exist in the same space as the gauge fields, whereas in string theory gravity emerges along with the extra dimension.

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