**Einstein's Succession, Part 5**(Other parts:

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**All cosmology described in the previous parts (****http://goo.gl/sSto2p****) is solely based on the Einstein field equations. It turns out, our universe not only seems to be flat, but could actually have zero total energy [23]. This leads to the speculation that it may have been created in a coincidental quantum fluctuation. Such a closed system would not require any higher structure to provide a trigger mechanism for the big bang. Yet predictions of quantum mechanics seem to contradict predictions from general relativity, and suddenly the picture becomes much more complicated.**Black holes for example are a solution of the Einstein field equations and describe a massive singularity surrounded by a gravitational field so strong that – within a well-defined surface known as the event horizon – even light cannot escape. Nevertheless, the quantum field theory predicts that black holes evaporate over time due to quantum vacuum fluctuations (creation of particle-antiparticle pairs of virtual particles) at the black hole’s event horizon. The escaping particle is known as Hawking radiation

**[25]** and causes the black hole to lose mass and energy. It can be shown with currently accepted theories that this particle must be entangled with its infalling antiparticle that is swallowed by the black hole, as well as with all the Hawking radiation previously emitted by the black hole

**[26]**. Since quantum mechanics forbids any particle to be fully entangled with two independent systems, the combination of general relativity, quantum mechanical unitarity, and quantum field theory creates a paradox

**[27]**. The very same Hawking radiation might even prevent black holes from forming in the first place

**[24]**, yet we know from observations that black holes do exist.

There is another—much simpler—gedanken experiment that also tells us that the universe cannot be described by Einsteins field equations alone: the horizon problem. Naturally, we can only retrieve information from within a certain volume that is defined by the cosmological horizon which represents the boundary of the observable universe. Due to the nature of an expanding universe, this horizon has a radius of 46.2 billion light years

**[28]** although light from that distance only traveled for 13.75 billion years (which represents the age of the universe). Light from outside this horizon had no chance to reach us yet. Thus it is obvious that we are embedded in a much larger unobservable structure that could have a different shape and where our local geometry only seems to be flat. Other parts of this larger structure, beyond our cosmological horizon, might host additional local universes of widely differing curvatures. The figure below shows Region A and Region B which both lie within our observable universe, but the local universes for both regions cover different parts of the overall universe and do not fully include each other. (Please note: These many local universes are very different to the parallel universes as a result of the many-worlds interpretation of quantum mechanics

**[29]** where new universes pop into existence for every possible outcome whenever an observation is made.)

This cosmological horizon imposes another paradox. We know that—due to its small size—the early universe was so dense and therefore so hot that photons scattered at free electrons. It took until 380,000 years after the big bang for protons and electrons to finally combine and form neutral hydrogen atoms. It was at this time that the universe became electromagnetically transparent. Today the light from this last scattering can be observed—now red shifted due to the cosmic expansion—in the cosmic microwave background (CMB) radiation. It tells us details about the conditions 380,000 years after the big bang. (This time represents the earliest direct observation currently possible as we have no way to observe the universe prior to that time.)

Due to the random nature of the initial conditions the temperature of this radiation should be very different for different directions in the sky. Similar to the figure below, distant regions in space back in the time of the last scattering had no causal contact. Light sent out from opposite patches of the origin of the CMB just reached Earth, which is positioned half way between them. Thus those patches can not know anything about each other. The distance between Earth and the origin of the CMB expanded to 46 billion light years. Thus even with a constant expansion of space at the current Hubble constant, light that just reached Earth could never reach the other side of the CMB: space in between Earth and the origin of the CMB expands so fast that both points seem to recede from each other faster than the speed of light. There is no change that an equilibrium was formed or could ever form and the initial temperature fluctuations should still be observable. Yet the CMB radiation has a surprisingly uniform temperature, isotropic to roughly one part in 100,000 over the entire sky, with a very fine fluctuation pattern that may have seeded the growth of structure in the universe. This cannot be explained by the standard ΛCDM model.

**Learn how scientists try to resolve this paradox in the next parts.** Subscribe to

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[23] L.M. Krauss.

*A Universe from Nothing: Why There Is Something Rather Than Nothing.* Atria Books, 2012.

[24] Laura Mersini-Houghton and Harald P Pfeier. Back-reaction of the

hawking radiation ux on a gravitationally collapsing star ii: Fireworks

instead of rewalls.

*arXiv preprint arXiv:1409.1837,* 2014.

[25] Stephen W Hawking. Black hole explosions.

*Nature,* 248(5443):30–31, 1974.

[26] Don N Page. Information in black hole radiation. _Physical review

letters,_ 71(23):3743, 1993.

[27] Ahmed Almheiri, Donald Marolf, Joseph Polchinski, and James Sully. Black holes: complementarity or rewalls?

*Journal of High Energy Physics,* 2013(2):1–20, 2013.

[28] J Richard Gott III, Mario Juric, David Schlegel, Fiona Hoyle, Michael Vogeley, Max Tegmark, Neta Bahcall, and Jon Brinkmann. A map of the universe.

*The Astrophysical Journal,* 624(2):463, 2005.

[29] Hugh Everett. The theory of the universal wave function. 1973.