The past hypothesis meets gravity

Callender, Craig

doi:10.1017/cbo9780511770777.003

Cited by 22 publications

(14 citation statements)

References 39 publications

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“…The change of thermal entropy of a gas of N molecules originally in thermal equilibrium at a temperature T, then separated into two parts with temperatures 1 T and 2 T with constant thermal energy so that       T T T . This decrease of entropy due to evolution away from thermal equilibrium is a second order effect in / T T .…”

Section: Entropy Change During Gravitational Contractionmentioning

confidence: 99%

“…Hence the universe must initially have been in a state of very low entropy. Callender [1] has discussed this "Past Hypothesis", writing: "The Boltzmann entropy of the entire universe was very low (compared to now) roughly 15 billion years ago. In particular, the entropy of this state was low enough to make OPEN ACCESS subsequent entropy increase likely for many billions of years."…”

Section: Introductionmentioning

confidence: 99%

“…Maybe gravity saves the Past Hypothesis. Callender [1] argues that this is far from obvious. In the present review we shall consider different aspects of this question.…”

Section: Introductionmentioning

confidence: 99%

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Entropy and Gravity

Grøn

2012

Entropy

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Abstract:The effect of gravity upon changes of the entropy of a gravity-dominated system is discussed. In a universe dominated by vacuum energy, gravity is repulsive, and there is accelerated expansion. Furthermore, inhomogeneities are inflated and the universe approaches a state of thermal equilibrium. The difference between the evolution of the cosmic entropy in a co-moving volume in an inflationary era with repulsive gravity and a matter-dominated era with attractive gravity is discussed. The significance of conversion of gravitational energy to thermal energy in a process with gravitational clumping, in order that the entropy of the universe shall increase, is made clear. Entropy of black holes and cosmic horizons are considered. The contribution to the gravitational entropy according to the Weyl curvature hypothesis is discussed. The entropy history of the Universe is reviewed.

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Section: Entropy Change During Gravitational Contractionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Entropy and Gravity

Grøn

2012

Entropy

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“…Heggie and Hut (2003)). (An excellent paper clearing up much of the confusion is Wallace (2010); see Callender (2009) for some problematic aspects of the statistical mechanics of gravitating systems and Padmanabhan (1990) for a mathematical treatment.) Some examples of common claims are: a) Boltzmann's statistical mechanics is not applicable to systems in which gravity is the dominant force.…”

Section: Entropy Of a Classical Gravitating Systemmentioning

confidence: 99%

Arrow(s) of Time without a Past Hypothesis

Lazarovici¹,

Reichert

2020

Statistical Mechanics and Scientific Explanation

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The paper discusses recent proposals by Carroll and Chen, as well as Barbour, Koslowski, and Mercati to explain the (thermodynamic) arrow of time without a Past Hypothesis, i.e. the assumption of a special (low-entropy) initial state of the universe. After discussing the role of the Past Hypothesis and the controversy about its status, we explain why Carroll's model -which establishes an arrow of time as typical -can ground sensible predictions and retrodictions without assuming something akin to a Past Hypothesis. We then propose a definition of a Boltzmann entropy for a classical N -particle system with gravity, suggesting that a Newtonian gravitating universe might provide a relevant example of Carroll's entropy model. This invites comparison with the work of Barbour, Koslowski, and Mercati that identifies typical arrows of time in a relational formulation of classical gravity on shape space. We clarify the difference between this gravitational arrow in terms of shape complexity and the entropic arrow in absolute spacetime, and work out the key advantages of the relationalist theory. We end by pointing out why the entropy concept relies on absolute scales and is thus not relational.

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“…First, the line of thought described above regarding equilibrium states for gravitational systems is heuristic, at best. Diffi culties with applying statistical mechanics to gravity arise even in Newtonian gravity and are amplifi ed in GR (see Callender 2008 ). And in assessing ever-earlier states, the appropriate physical theory to employ to determine the equilibrium states -namely, quantum gravity -has not yet been formulated.…”

Section: Time ' S Arrowmentioning

confidence: 99%

Time in Cosmology

Smeenk¹

2013

A Companion to the Philosophy of Time

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Modern cosmology apparently answers many longstanding questions regarding the nature of time. 1 For example, is the universe temporally fi nite or eternal, and is there a unique global sense of time? The Standard Model of cosmology renders the following verdicts. The universe is temporally fi nite, and approximately 13.7 billion years old. Events in the universe can be ordered according to a "cosmic time," which corresponds to time as measured by a particular class of fundamental observers since the "Big Bang." Many questions that remain unanswered seem to be answerable by empirical methods, if only in principle. Whether the universe will also be temporally fi nite to the future can be resolved, in principle, by measuring the actual matter and energy density in the universe. If it exceeds the so-called critical value, then the universe will be temporally fi nite to the future as well, collapsing back into a "Big Crunch."Taking relativistic cosmology to provide direct answers to such questions is, however, misleading in (at least) two different senses. First, it neglects the extent to which the questions have not been so much answered as transformed into, or replaced with, new questions. A similar transformation occurred with regard to the central cosmological question of the sixteenth and seventeenth centuries: does the Sun or the Earth move? Posing this question suffi ciently clearly for astronomical evidence to provide a decisive resolution of it required reformulating the concepts of space, time, and motion. Similarly, aspects of time presumed in posing the questions above pre-theoretically are signifi cantly revised in relativistic cosmology. Second, even taking these conceptual changes into account, the answers to the questions are subtler than is usually acknowledged. The answers summarized above hold for a class of particularly simple, idealized cosmological models. But in what sense do these answers apply to the real universe? And in what sense do these answers address the initial questions regarding the nature of time?This chapter aims to provide a self-contained introduction to time in relativistic cosmology that clarifi es both how questions about the nature of time should be posed 13 A Companion to the Philosophy of Time, First Edition. Edited by Heather Dyke and Adrian Bardon.

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The past hypothesis meets gravity

Cited by 22 publications

References 39 publications

Entropy and Gravity

Entropy and Gravity

Arrow(s) of Time without a Past Hypothesis

Time in Cosmology

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