3.3 What prospects for Cosmology?

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Philosophy and the Sciences: Introduction to the Philosophy of Physical Sciences

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The University of Edinburgh

Philosophy and the Sciences: Introduction to the Philosophy of Physical Sciences

369 個評分

What is the origin of our universe? What are dark matter and dark energy? This is the first part of the course 'Philosophy and the Sciences', dedicated to Philosophy of the Physical Sciences. Scientific research across the physical sciences has raised pressing questions for philosophers. The goal of this course is to introduce you to some of the main areas and topics at the key juncture between philosophy and the physical sciences. Each week we will introduce you to some of these important questions at the forefront of scientific research. We will explain the science behind each topic in a simple, non-technical way, while also addressing the philosophical and conceptual questions arising from it. We’ll consider questions about the origin and evolution of our universe, the nature of dark energy and dark matter and the role of anthropic reasoning in the explanation of our universe. Learning Objectives Gain a fairly well-rounded view on selected areas and topics at the intersection of philosophy and the sciences Understand some key questions, and conceptual problems arising in the natural sciences. Develop critical skills to evaluate and assess these problems. Suggested Reading To accompany 'Philosophy and the Sciences', we are pleased to announce a tie-in book from Routledge entitled 'Philosophy and the Sciences for Everyone'. This course companion to the 'Philosophy and the Sciences' course was written by the Edinburgh Philosophy and the Sciences team expressly with the needs of MOOC students in mind. 'Philosophy and the Sciences for Everyone' contains clear and user-friendly chapters, chapter summaries, glossary, study questions, suggestions for further reading and guides to online resources. Please note, this companion book is optional - all the resources needed to complete the course are available freely and listed on the course site.

從本節課中

Week 3: Dark Matter and Dark Energy (Michela Massimi and John Peacock)

According to the currently accepted model in cosmology, our universe is made up of 5% of ordinary matter, 25% cold dark matter, and 70% dark energy. But what kind of entities are dark matter and dark energy?

3.1 Introduction8:08

3.2 Dark Matter & Dark Energy part I18:01

3.2 Dark Matter & Dark Energy part II13:33

3.3 What prospects for Cosmology?9:07

3.4 Conclusion3:28

與講師見面

Professor Michela Massimi
Full Professor
Philosophy
Dr. Alasdair Richmond
Dr.
Philosophy
Dr. Suilin Lavelle
Lecturer in Philosophy
University of Edinburgh
Dr David Carmel
Lecturer
Psychology
Dr Mark Sprevak
Senior Lecturer
School of Philosophy, Psychology and Language Sciences
Professor Duncan Pritchard
Professor of Philosophy
University of Edinburgh
Professor Andy Clark

School of Philosophy, Psychology and Language Sciences
Professor John Peacock
Professor of Cosmology
Institute for Astronomy
Professor Barbara Webb

School of Informatics
Dr Kenny Smith

School of Philosophy, Psychology & Language Sciences
Dr Peggy Series

Institute for Adaptive and Neural Computation
Louise Connelly
E-Learning Developer
University of Edinburgh

So, going then back to the underdetermination problem,

we may ask what is the available evidence for the concordance model?

And here we will find replicated Pierre Duhem's story about how a piece of negative

evidence may not necessarily speak against the main theoretical hypothesis,

but may instead indicate the need of introducing an auxiliary hypothesis.

If that's the case, then the problem of underdetermination of theory by

evidence looms on the horizon as the following table shows.

So if the underdetermination argument is correct,

then the choice between the concordance model and

some of its rivals must seem to be not on rational ground.

And one might be tempted to bring in a sociological explanation of

why scientists gather their consensus around the concordance model and

the hypothesis of dark energy and dark matter.

For example, one might argue that it's easier to hold on to a well

established and well accepted theory such as general relativity

plus the auxiliary hypothesis of dark energy than trying to modify the theory.

And even in those cases where we do have a very well worked out alternative such

as MOND, people may prefer to hold on to traditional Newtonian dynamics because of

the ad-hoc nature of the modification of the Newtonian dynamics required by MOND.

Are scientists following their good sense in taking those decisions, and

what counts as good sense?

For example, a MOND supporter would argue that they have a very successful

theory to explain galaxy rotation curves without resorting to dark matter.

From their point of view, good sense would recommend accepting

relativistic MOND over the concordance model.

To give an answer to these pressing questions at the forefront of

contemporary research in cosmology, we should go back to one of the premises of

the underdetermination argument, the premise about empirical equivalence, and

ask whether we do have genuine empirically equivalent competitors for

dark energy and dark matter.

For example, one rival dark energy-free model is the so-called

Inhomogeneous Lemaitre-Tolman-Bondi, or LTB model, rather than the FLRW

models of the concordance model, which assumes, with the Cosmological Principle,

that our universe is roughly homogenous and isotropic.

Namely, it assumes that our universe has the same

uniform structure in all spatial positions and in all spatial directions.

So if we deny homogeneity but retain isotropy, we might be assuming that there

are variations, spatial variations in the distribution of matter in our universe.

And we may want to assume that we occupy an underdense, or

void region in our universe, what cosmologists call the Hubble Bubble,

that is accelerating at a rate faster than the average.

So the argument goes that our inference to dark energy is flawed because

it's based on a flawed homogeneous FLRW model.

Critics of dark energy argue that an LTB model is a genuine empirically equivalent

rival to FLRW model, because it can account for, for example, fluctuations

in cosmic microwave background, as well as accounting for the same evidence

that dark energy typically explains, namely supernova 1a data.

Although with an important caveat.

Supernova 1a data, which is a typical evidence for

an accelerating expansion of the universe for

which dark energy is usually introduced, is now interpreted without acceleration.

Dark energy critics argue that light traveling through an inhomogeneous

universe doesn't see the Hubble expansion, and we make inferences about

an accelerating expansion using red shift and light intensity, neither of which can

track any eventual inhomogeneities in the universe through which the light travels.

However, many cosmologists find LTB models unattractive because of the need to place

ourselves in a very special position in our universe, a so-called Hubble Bubble.

And this violates what is called the Copernican principle, namely,

the view named after Copernicus that we don't occupy any special or

privileged position in our universe.

Moreover, one may retort that the charge of

being ad hoc affects the interpretation of supernova 1a data here as

implying no acceleration no less that it affects the assumption of dark energy,

the assumption of a known zero vacuum energy density to interpret the very

same data as data for an accelerating expansion of the universe.

What if instead of modifying FLRW models,

we try to modify general relativity instead, and go with dark energy?

Well, some physicists appeal to string theory to speculate that there might be

many vacua with different energy of the vacuum density, and

we might just be occupying a very special vacuum

with a tiny positive value of the vacuum energy density.

But it's fair to say,

that as of today, we don't really have a genuine contender to general relativity.

Shifting to rivals of dark matter, the best candidate we currently have is

Modified Newtonian Dynamics, or MOND, first proposed by

Milgrom in the 1980's and in its relativistic form by Bekenstein.

As we mentioned before, MOND was introduced to explain the anomaly of

galaxy flat rotation curves without introducing the auxiliary

hypothesis of dark matter, but by modifying Newton's law instead.

MOND supporters appeal to arguments from simplicity and mathematical elegance.

For example, Bekenstein argues that for disk galaxies MOND provides

a more economical and more falsifiable theory than the dark matter paradigm.

It's interesting that appeals should be made here to Popper's falsifiability and

simplicity as arguments for preferring MOND over dark matter,

which brings us back to the philosophical topic of today's class.

Do we have genuine empirically equivalent rivals to dark energy and dark matter?

Or is the theory choice between dark energy and

dark matter on one hand and rivals really underdetermined by evidence.

Was Thomas Kuhn right in thinking that neither simplicity nor

any other criteria will ever be sufficient to

explain how scientists go about making decisions about which theory to support?

As a philosophical reply to those questions, one might stress that there is

a lot more to theory choice than just the ability of two rival theories to imply or

to entail the same piece of evidence.

It is within this context that philosophers of science appeal to

the notion of empirical support as a way forward in the debate about

underdetermination of theory by evidence and the rationality of theory choice.

For example, the concordance model is not just empirically supported by direct

empirical evidence that we may be able to find one day about dark energy and

dark matter.

But the model is embedded into a larger theoretical framework, general relativity.

And in so doing, the model receives indirect empirical support from any other

piece of evidence that is a consequence of

the larger theoretical framework within which the model is embedded.

Along similar lines, although none of the phenomena of

terrestrial mechanics is directly relevant to galaxy flat rotation curves,

one may appeal to the success of Newtonian dynamics across this wide

range spectrum of phenomena as an argument for Newtonian dynamics being

empirically very well supported, and not in need of any ad-hoc modification,

such as those required by MOND, to explain anomalous phenomena in cosmology.

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