The size of our causal Universe

ABSTRACT A Universe with finite age also has a finite causal scale. Larger scales cannot affect our local measurements or modelling, but far away locations could have different cosmological parameters. The size of our causal Universe depends on the details of inflation and is usually assumed to be larger than our observable Universe today. To account for causality, we propose a new boundary condition, that can be fulfill by fixing the cosmological constant (a free geometric parameter of gravity). This forces a cancellation of vacuum energy with the cosmological constant. As a consequence, the measured cosmic acceleration cannot be explained by a simple cosmological constant or constant vacuum energy. We need some additional odd properties such as the existence of evolving dark energy (DE) with energy-density fine tuned to be twice that of dark matter today. We show here that we can instead explain the current cosmic acceleration without DE (or modified gravity) as a the result of a primordial inflation with a causal scale smaller than the observable Universe today. Such scale corresponds to half the sky at z = 1 and 60 deg at z= 1100, which is consistent with the anomalous lack of correlations observed in the CMB.

Dark matter in logarithmic F(R) gravity

The logarithmic [Formula: see text]-corrected [Formula: see text] gravity is investigated as a prototype model of modified gravity theories with quantum corrections. By using the auxiliary field method, the model is described by the general relativity with a scalaron field. The scalaron field can be identified as an inflaton at the primordial inflation era. It is also one of the dark matter candidates in the dark energy (DE) era. It is found that a wide range of the parameters is consistent with the current observations of CMB fluctuations, DE and dark matter.

The Case for Nonlocal Modifications of Gravity

The huge amounts of undetected and exotic dark matter and dark energy needed to make general relativity work on large scales argue that we should investigate modifications of gravity. The only stable, metric-based and invariant alternative to general relativity is f(R) models. These models can explain primordial inflation, but they cannot dispense with either dark matter or dark energy. I advocate nonlocal modifications of gravity, not as new fundamental theories but rather as the gravitational vacuum polarization engendered by infrared quanta produced during primordial inflation. I also discuss some of the many objections which have been raised to this idea.

The Dark Side

Chapter 6 deals with the remaining mysteries in cosmology—dark matter, dark energy, and inflationary expansion—and the experiments aimed at solving them. It reviews the evidence for dark matter, and experiments to detect the microscopic particles proposed as its constituents: weakly interacting massive particles and invisible axions. Contrasts are drawn between the failure to understand the scale of dark energy theoretically and the ambitious new survey telescopes, such as the Large Synoptic Survey Telescope (or LSST), that aim to constrain its equation of state. The theoretical concepts and possible experimental signatures of cosmic inflation are described. Searches for possible imprints from primordial inflation-induced gravitational waves on the polarization of the cosmic microwave background (CMB polarization) are discussed in the context of the pioneering first detection by the Laser Interferometer Gravitational-Wave Observatory (or LIGO) of gravitational waves from distant black-hole mergers. Philosophical questions regarding the falsifiability of inflation are raised.