It was then used to support claims that, just as the entire universe orbits the Sun, so the whole of society orbits the king [ 26 ]. Propaganda in support of absolute monarchy then reached its height in the iconography of the French king Louis XIV. Such authoritarian ideas were directly countered by what I have called Political Newtonianism [ 22 ]. This held that, just as Newton had argued that one law governed the whole universe, so the same principle must apply to terrestrial affairs and one law must govern the whole of human society.
In principle, then, all people were equal, and there was no justification for monarchy. Such ideas were influential among both the American revolutionaries of [ 27 ] and the French in Newtonianism was taken one step further by Auguste Comte — , the founder of sociology.
Adding Galileo and Kepler to Newton as sources of authority, Comte [ 28 ] argued that if the entire universe was a mathematically regulated mechanism, so human society must also be governed by the same principle. If planets moved in mathematically determined patterns, Comte reasoned, so must people. By collecting and analysing data on human behaviour, Comte concluded, the same laws that controlled the wider universe should be discovered in human affairs.
There have been a few attempts, for example, to identify Einsteinian relativity either as a form of political discourse, or to draw political implications from it. As far as the former is concerned, I refer to the French feminist and social theorist Luce Irigaray who has identified the theory of relativity as a political rather than scientific formula [ 29 ].
Sokal and Bricmont [ 28 ], meanwhile, noted how the notion of relativity in time and space was used by postmodern theorists in order to advocate cultural relativity on the grounds that, if the universe has no single centre, neither does culture. The consequences of Newtonianism the belief that the entire universe is mathematically regulated permeate western thought wherever there has been a search for a universal law based on supposedly hard data.
Psychology is a prime example. The measurement and mathematical analysis of the human mind then became the basis of much psychiatry and academic psychology. Jung opened a radically different strand of thought in modern psychology which is highly influential in many schools of psychotherapy and counselling practiced in society as a whole, although usually outside the academic system.
Jung revived the Platonic theory that everything in the world is a manifestation of an original pure idea or archetype. The idea that one can become one who truly is also relates back to Aristotelian cosmology in which it was thought that the four elements fire, earth, air and water all try to find their natural place in the world.
This, Aristotle thought, was why flames go up to the sky, where fire belongs, and water falls to the ground, because that is where it finds its natural home. In Aristotelian politics, kings are at the top of society and peasants at the bottom, because that is the natural state of affairs; in Aristotelian psychology every individual then has a natural way of being.
Jung, though, was equally concerned with the latest science, and formed a collaboration and friendship with the quantum physicist Wolfgang Pauli — [ 35 ]. Together they formulated the concept of synchronicity by which meaningful events are connected because they take place at the same time, without any causal connection [ 36 ].
Newtonian psychology—the belief that all mental states can be measured—survives in university departments and psychiatry. Like Dante, Godwin used his story to describe the structure of the cosmos, now, after Johannes Kepler and Galileo, rejecting the planetary spheres and challenging the Aristotelian idea that all things have their natural place.
Godwin departed from the old idea of a journey of the soul or a dream world. Instead, his hero, Gonzales, flies to the Moon carried by giant geese. While celestial journey films can be enjoyed as simple adventures, they often contain deeper meanings. Released at a time when the Cold War was reaching its height with the conflict in Korea, the story featured a wise alien who arrived from space in order to reveal to humanity the error of its ways.
Cosmology, through film, then becomes a means of commenting on societal change. It is set in a utopian future in which there is one world government, collaborating with other worlds through United Federation of Planets, and money has been abolished. The values espoused by the Federation were American: freedom from tyranny, freedom of expression and respect for minorities. In all such cases the cosmos is seen as a blank slate, a tabula rasa, on which human concerns are imposed.
There is a constant strand of literary comment on cosmology in the nineteenth century. Poe realised that in a Newtonian universe the stars are likely to collapse in on each other, and that therefore the universe must be evolving [ 42 ]. For Hardy evolution was a reality. His portrayal of Galileo as the heroic intellectual defending Copernicus, struggling against an obscurantist Inquisition inaccurate because many in the senior Catholic hierarchy were Copernicans , was an allegory of the revolutionary struggles of the s.
This hypothetical particle, the tachyon, might as Martin Rees [ 46 ], says alter the order of events, if a signal from a tachyon arrived before it was sent. The Doctor himself is increasingly represented as a lonely figure, destined to exist in perpetual sadness caused by the death or departure of his companions.
Raindrops hang motionless in air. The concept that all time exists simultaneously actually has a long lineage. Augustine V. Elliot, impressed by Einstein, combined the lessons of relativity with Plato, Ecclesiastes and Augustine. In two poems composed in the s, and published in —, Elliot considered the conundrum of time for the human condition.
The popular end of such speculation is best represented by the collected works of Philip K. But the SM is far from complete, and there are three different types of significant open issues.
First, we do not understand three crucial components of the SM that require new physics. We do not have a full account of the nature, or underlying dynamics, of dark matter Bertone et al. The second set of open questions regards structure formation.
While the account of structure formation matches several significant observed features, such as the correlations among galaxies in large scale surveys, there are a number of open questions about how galaxies form Silk Many of these, such as the cusp-core problem Weinberg et al. This is also a very active area of research, driven in particular by a variety of new lines of observational research and large-scale numerical simulations.
The third and final set of open issues regards possible observations that would show that the SM is substantially wrong. Any scientific theory should be incompatible with at least some observations, and that is the case for the SM. In the early days of relativistic cosmology, the universe was judged to be younger than some stars or globular clusters.
This conflict arose due to a mistaken value of the Hubble constant. There is currently no such age problem for the SM, but obviously discovering an object older than Although cosmology is generally seen as fitting into the general physics paradigm of everything being determined in a bottom up manner, as in the discussion above, there is another tradition that sees the effect of the global on the local in cosmology. In each case, global boundary conditions have an important effect on local physics.
More recent ones relate to. This is the part of the universe that actually has a significant effect on our history. Many philosophers hold that evidence is not sufficient to determine which scientific theory we should choose.
Scientific theories make claims about the natural world that extend far beyond what can be directly established through observations or experiments.
Rival theories may fare equally well with regard to some body of data, yet give quite different accounts of the world. The nature of this proposed underdetermination of theory by evidence, and appropriate responses to it, have been central topics in philosophy of science Stanford [].
Although philosophers have identified a variety of distinct senses of underdetermination, they have generally agreed that underdetermination poses a challenge to justifying scientific theories. There is a striking contrast with discussions of underdetermination among scientists, who often emphasize instead the enormous difficulty in constructing compelling rival theories. Suppose that the empirical content of theory consists of a set of observational claims implied by the theory.
Philosophers then take the existence of rival theories to be straightforward. If we demand more of theories than empirical adequacy in this sense, it is possible to draw distinctions among theories that philosophers would regard as underdetermined. Instead, they differ in various ways: intended domain of applicability, explanatory scope, importance attributed to particular problems, and so on.
Given a more stringent account of empirical success it is much more challenging to find rival theories. One aspect of underdetermination emphasized by Stanford is of more direct relevance to scientific debates: current theories may be indistinguishable, within a restricted domain, from a successor theory, even though the successor theory makes different predictions for other domains. This raises the question of how far we can rely on extrapolating a theory to a new domain.
For example, despite its success in describing objects moving with low relative velocities in a weak gravitational field, where it is nearly indistinguishable from general relativity, Newtonian gravity does not apply to other regimes. How far, then, can we rely on a theory to extend our reach? The obstacles to making such reliable inferences reflect the specific details of particular domains of inquiry.
Below we will focus on the obstacles to answering theoretical questions in cosmology due to the structure of the universe and our limited access to phenomena. Given the grand scope of cosmology, one might expect that many questions must remain unresolved. Basic features of the SM impose two fundamental limits to the ambitions of cosmological theorizing.
First, the finitude of the speed of light ensures that we have a limited observational window on the universe due to existence of the visual horizon, representing the most distant matter from which we can receive and information by electromagnetic radiation, and the particle horizon, representing the most distant matter with which we can have had any causal interaction matter up to that distance can influence what we see at the visual horizon.
Recent work has precisely characterized what can be established via idealized astronomical observations, regarding spacetime geometry within, or outside, our past light cone the observationally accessible region.
Second, in addition to enormous extrapolations of well-tested physics in the SM, cosmologists have explored speculative ideas in physics that can only be tested through their implications for cosmology; the energies involved are too high to be tested by any accelerator on Earth. We will briefly discuss how this second type of horizon poses limits for cosmological theorizing. In both cases, the type of underdetermination that arises differs from that discussed in the philosophical literature.
To what extent can observations determine the spacetime geometry of the universe directly? By contrast, the standard approach starts by assuming a background cosmological model and then finding an optimal parameter fit. Roughly put, the ideal data set consists of a set of astrophysical objects that can be used as standard candles and standard rulers. If the intrinsic properties and evolution of a variety of sources are given, observations can directly determine the area or luminosity distance of the sources, and the distortion of distant images determines lensing effects.
Number counts of discrete sources such as galaxies or clusters can be used to infer the total amount of baryonic matter, again granting various assumptions. Ellis et al. Unless this is the case, the causal past for a single observer, and even a collection of causal pasts, place very weak constraints on the global properties of spacetime. The global properties of a spacetime characterize its causal structure, such as the presence or absence of singularities.
One way to make this question precise is to consider whether there are any global properties shared by spacetimes that are constructed as follows. It is possible, however, to construct counterparts that do not have the same global properties as the original spacetime. The property of having a Cauchy surface, for example, need not be shared by an indistinguishable counterpart. This line of work establishes that some global properties cannot be established observationally, and raises the question of whether there are alternative justifications.
The case of global spacetime geometry is not a typical instance of underdetermination of theory by evidence, as discussed by philosophers, for two reasons see Manchak , Norton , Butterfield First, this whole discussion assumes that classical GR holds; the question regards discriminating among models of a given theory, rather than a choice among competing theories.
Second, these results establish that all observations available to us that are compatible with a given spacetime, with some appealing global property, are equally compatible with its indistinguishable counterparts. But as is familiar from more prosaic examples of the problem of induction, evidence of past events is compatible, in a similar sense, with many possible futures. Standard accounts of inductive inference aim to justify some expectations about the future as more reasonable, e.
The challenge in this case is to articulate an account of inductive inferences that justifies accepting one spacetime over its indistinguishable counterparts. As a specific instance of this challenge, consider the status of the cosmological principle, the global symmetry assumed in the derivation of the FLRW models. The results above show that all evidence available to us is equally compatible with models in which the cosmological principle does or does not hold. One might take the principle as holding a priori , or as a pre-condition for cosmological theorizing Beisbart A recent line of work aims to justify the FLRW models by appealing to a weaker general principle in conjunction with theorems relating homogeneity and isotropy.
Global isotropy around every point implies global homogeneity, and it is natural to seek a similar theorem with a weaker antecedent formulated in terms of observable quantities.
The Ehlers-Geren-Sachs theorem Ehlers et al. Other tests are direct tests with a good enough set of standard candles, and an indirect test based on the time drift of cosmological redshift. This line of work provides an empirical argument that the observed universe is well-approximated by an FLRW model, thus changing that assumption from a philosophically based starting point to an observationally tested foundation. But cosmologists have pursued a variety of questions that extend beyond these core theories.
In these domains, cosmologists face a form of underdetermination: should a phenomena be accounted for by extending the core theories, or by changing physical or astrophysical assumptions?
For many aspects of fundamental physics, including quantum gravity in particular, cosmology provides the only feasible way to assess competing ideas.
This ambitious conception of cosmology as the sole testing ground for new physics extends beyond the standard model of particle physics which is generally thought to be incomplete, even though there are no observations that contradict it. Big bang nucleosynthesis, for example, is an application of well-tested nuclear physics to the early universe, with scattering cross-sections and other relevant features of the physics fixed by terrestrial experiments.
While working out how nuclear physics applied in detail required substantial effort, there was little uncertainty regarding the underlying physics. The horizon can be characterized more precisely for a chosen theory, by specifying the regions of parameter space that can be directly tested by experiments and observations.
This is not to deny that there may be strong theoretical grounds to favor particular proposals, as extensions of the core theories. Cosmological physics extending beyond the physics horizon faces an underdetermination threat due to the lack of independent lines of relevant evidence. The case of dark matter illustrates the value of such independent evidence. Dark matter was first proposed to account for the dynamical behavior of galaxy clusters and galaxies, which could not be explained using Newtonian gravitational theory with only the luminous matter observed.
Dark matter also plays a crucial role in accounts of structure formation, as it provides the scaffolding necessary for baryonic matter to clump, without conflicting with the uniformity of the CMB.
There is an active research program MOND, for Mo dified N ewtonian D ynamics devoted to accounting for the relevant phenomena by modifying gravity. Efforts have been underway for some time to find dark matter particles through direct interactions with a detector, mediated by the weak force. A positive outcome of these experiments would provide evidence of the existence of dark matter that does not depend upon gravitational theory.
Such independent evidence is not available for two prominent examples of new physics motivated by discoveries in cosmology. Subsequent observations of the redshift-distance relation, using supernovae type Ia as a standard candle, led to the discovery that the expansion of the universe is accelerating. Unlike dark matter, however, the properties of dark energy insure that any attempt at non-cosmological detection would be futile: the energy density is so small, and uniform, that any local experimental study of its properties is practically impossible.
Inflationary cosmology originally promised a powerful unification of particle physics and cosmology. The earliest inflationary models explored the consequences of specific scalar fields introduced in particle physics the then supposed Higgs field for the strong interactions.
If the properties of the inflaton field are unconstrained, inflationary cosmology is extremely flexible; it is possible to construct an inflationary model that matches any chosen evolutionary history of the early universe. In principle, cosmological observations could determine some of the properties of the inflaton field and so select among them Martin et al.
This could in principle then have implications for a variety of other experiments or observations; yet in practice the features of the inflaton field in most viable models of inflation guarantee that it cannot be detected in other regimes.
The physics horizon poses a challenge because one particularly powerful type of evidence—direct experimental detection or observation, with no dependence on cosmological assumptions—is unavailable for the physics relevant at earliest times before inflation, and indeed even for baryosynthesis after inflation.
Yet this does not imply that competing theories, such as dark matter vs. The case in favor of dark matter draws on diverse phenomena, and it has been difficult to produce a compelling modified theory of gravity, consistent with GR, that captures the full range of phenomena as an alternative to dark matter. Cosmology typically demands a more intricate assessment of background assumptions, and the degree of independence of different tests, in evaluating proposed extensions of the core theories.
There is a distinctive form of underdetermination regarding the use of statistics in cosmology, due to the uniqueness of the universe. To compare the universe with the statistical predictions of the SM, we conceptualize it as one realization of a family of possible universes, and compare what we actually measure with what is predicted to occur in the ensemble of hypothetical models. When they are significantly different, the key issue is: Are these just statistical fluctuations we can ignore?
Or are they serious anomalies that need an explanation? How do we decide? This will depend on the particular measurement see e. In all the physical sciences, this is a unique problem of cosmology. Cosmology confronts a distinctive challenge in accounting for the origin of the universe. In most other branches of physics the initial or boundary conditions of a system do not call out for theoretical explanation.
They may reflect, for example, the impact of the environment, or an arbitrary choice regarding when to cut off the description of a subsystem of interest. This basic question regarding the nature of aims of a theory of origins has significant ramifications for various lines of research in cosmology.
Contemporary cosmology at least has a clear target for a theory of origins: the SM describes the universe as having expanded and evolved over Past singularities, signaled by the existence of inextendible geodesics with bounded length, must be present in models with a number of plausible features. Geodesics are the curves of extreme length through curved spacetime, and freely falling bodies follow timelike geodesics.
The singularity theorems plausibly apply to the observed universe, within the domain of applicability of general relativity. There are various related theorems differing in detail, but one common ingredient is an assumption that there is sufficient matter and energy present to guarantee that our past light cone refocuses. The theorems also require an energy condition: a restriction on the types of matter present in the model, guaranteeing that gravity leads to focusing of nearby geodesics.
In eqn. The prediction of singularities is usually taken to be a deep flaw of GR. This concern only applies to some kinds of singularities, however. Relativistic spacetimes that are globally hyperbolic have Cauchy surfaces, and appropriate initial data posed on such surfaces fix a unique solution throughout the spacetime. Global hyperbolicity does not rule out the existence of singularities, and in particular the FLRW models are globally hyperbolic in spite of the existence of an initial singularity.
In any case, it is clear that the presence of a singularity in a cosmological model indicates that spacetime, as described by GR, comes to an end: there is no way of extending the spacetime through the singularity, without violating mathematical conditions needed to insure that the field equations are well-defined. There are two limitations regarding what we can learn about the origins of the universe based on the singularity theorems.
First, although these results establish the existence of an initial singularity, they do not provide much guidance regarding its structure. Partial results have been established for restricted classes of solutions; for example, numerical simulations and a number of theorems support the BKL conjecture, which holds that isotropic, inhomogeneous models exhibit a complicated form of chaotic, oscillatory behavior.
The resulting picture of the approach to the initial singularity contrasts sharply with that in FLRW models. Second, classical general relativity does not include quantum effects, which are expected to be relevant as the singularity is approached. Crucial assumptions of the singularity theorems may not hold once quantum effects are taken into account. The standard energy conditions do not hold for quantum fields, which can have negative energy densities. On this account, GR fails to provide a good approximation in the region of the bounce, and the apparent singularity is an artifact.
There are several accounts of the early universe, motivated by string theory and other approaches, that similarly avoid the initial singularity due to quantum gravity effects. For example, this might be taken as the state specified on a spatial hypersurface at a very early cosmic time. However, the domain of applicability of GR is not well understood, given uncertainty about quantum gravity. Projecting observed features of the universe backwards leads to an initial state with three puzzling features: [ 35 ].
The theory of inflation discussed below aims to explain these issues. There are three main approaches to theories of the initial state, all of which have been pursued by cosmologists since the late 60s in different forms. Expectations for what a theory of initial conditions should achieve have been shaped, in particular, by inflationary cosmology. Inflation provided a natural account of the three otherwise puzzling features of the initial state emphasized in the previous section.
The first approach aims to reduce dependence on special initial conditions by introducing a phase of attractor dynamics. During an inflationary stage, arbitrary initial states are claimed to converge towards a state with the three features described above.
The second approach regards the initial state as extremely special rather than generic. Penrose treats the second law as arising from a law-like constraint on the initial state of the universe, requiring that it has low entropy. It does not account for the observed perturbations, however. Later he proposed the idea of Conformal Cyclic Cosmology, where such a special initial state at the start of one expansion epoch is the result of expansion in a previous epoch that wiped out almost all earlier traces of matter and radiation Penrose There are still, of course, questions regarding the initial state of the multiverse ensemble, if one exists.
For example, an inflationary stage can only begin in a region of spacetime if the inflaton field and the geometry are uniform over a sufficiently large region, such that the stress-energy tensor is dominated by the potential term implying that the derivative terms are small and the gravitational entropy is small. There are other model-dependent constraints on the initial state of the inflaton field. The majority of those working in inflationary cosmology instead appeal to the third approach: rather than treating inflation as an addition to standard big-bang evolution in a single universe, we should treat the observed universe as part of a multiverse, discussed below.
But even this must have a theory of initial conditions. Cosmology provokes questions about the limits of scientific explanation because it lacks many of the features that are present in other areas of physics.
Physical laws are usually regarded as capturing the features of a type of system that remain invariant under some changes, and explanations often work by placing a particular event in larger context. Theories of the initial state cannot appeal to either idea: we have access to only one universe, and there is no larger context to appeal to in explaining its properties. This contrast between the types of explanation available in cosmology and other areas of physics has often led to dissatisfaction see, e.
At the very least, cosmology forces us to reconsider basic questions about modalities, and what constitutes scientific explanation.
Though he was proven wrong about the steady state theory, Hoyle is credited with the major developments in the theory of stellar nucleosynthesis , which is the theory that hydrogen and other light atoms are transformed into heavier atoms within the nuclear crucibles called stars, and spit out into the universe upon the star's death. These heavier atoms then go on to form into water, planets, and ultimately life on Earth, including humans!
Thus, in the words of many awestruck cosmologists, we are all formed from stardust. Anyway, back to the evolution of the universe. As scientists gained more information about the universe and more carefully measured the cosmic microwave background radiation, there was a problem.
As detailed measurements were taken of astronomical data, it became clear that concepts from quantum physics needed to play a stronger role in understanding the early phases and evolution of the universe.
This field of theoretical cosmology, though still highly speculative, has grown quite fertile and is sometimes called quantum cosmology. Quantum physics showed a universe that was pretty close to being uniform in energy and matter but wasn't completely uniform. However, any fluctuations in the early universe would have expanded greatly over the billions of years that the universe expanded So cosmologists had to figure out a way to explain a non-uniform early universe, but one which had only extremely small fluctuations.
Enter Alan Guth, a particle physicist who tackled this problem in with the development of inflation theory. The fluctuations in the early universe were minor quantum fluctuations, but they rapidly expanded in the early universe due to an ultra-fast period of expansion. Astronomical observations since have supported the predictions of the inflation theory and it is now the consensus view among most cosmologists.
Though cosmology has advanced much over the last century, there are still several open mysteries. In fact, two of the central mysteries in modern physics are the dominant problems in cosmology and astrophysics:. There are some other suggestions to explain these unusual results, such as Modified Newtonian Dynamics MOND and variable speed of light cosmology, but these alternatives are considered fringe theories that are not accepted among many physicists in the field.
It is worth noting that the big bang theory actually describes the way the universe has evolved since shortly after its creation, but cannot give any direct information about the actual origins of the universe. This isn't to say that physics can tell us nothing about the origins of the universe. When physicists explore the smallest scale of space, they find that quantum physics results in the creation of virtual particles, as evidenced by the Casimir effect.
In fact, inflation theory predicts that in the absence of any matter or energy, then spacetime would expand. Taken at face value, this, therefore, gives scientists a reasonable explanation for how the universe could initially come into being. If there were a true "nothing", no matter, no energy, no spacetime, then that nothing would be unstable and would begin generating matter, energy, and an expanding spacetime.
This is the central thesis of books such as The Grand Design and A Universe From Nothing , which posit that the universe can be explained without reference to a supernatural creator deity. It would be hard to over-emphasize the cosmological, philosophical, and perhaps even theological importance of recognizing that the Earth was not the center of the cosmos.
In this sense, cosmology is one of the earliest fields that yielded evidence that was in conflict with the traditional religious worldview. In fact, every advance in cosmology has seemed to fly in the face of the most cherished assumptions that we'd like to make about how special humanity is as a species This passage from The Grand Design by Stephen Hawking and Leonard Mlodinow eloquently lays out the transformation in thinking that has come from cosmology:.
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