Dr Martin Hendry Dept of Physics and Astronomy University of Glasgow

A little old lady at the back of the room got up and said: What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise. The scientist gave a superior smile before replying What is the tortoise standing on?

Science: The intellectual and practical activity encompassing the systematic study of the structure and behaviour of the physical and natural world through observation and experiment From the New Oxford dictionary

Bernard Carr

Two talks in one: Why are we here?... Why are we here?...

Why are we here?...

Edwin Hubble

Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.

Hubble s s Law Distant galaxies are moving away from us with a speed proportional to their distance

Spacetime is expanding like the surface of a balloon. As the balloon expands, galaxies are carried farther apart

How fast is the Universe expanding?

What is driving the cosmic acceleration?

CMBR: all-sky temperature map from WMAP satellite. Temperature fluctuations from ~380,000 years after the Big Bang

Galaxies and Cosmology: the Basic Paradigm CMBR fluctuations, are the seeds of today s galaxies Structure we see today was assembled by gravity, causing density fluctuations in the early Universe (which we see imprinted on the CMBR) to grow, as the background smooth Universe expands

So what exactly is this dark energy?... Einstein s cosmological constant?... Energy of the quantum vacuum?...

State of the Universe Apr 2008 4% 22% Atoms Cold Dark Matter Dark Energy 74%

State of the Universe Apr 2008 4% 22% Ω b Ω CDM Why does 96% of the Ω Universe consist of strange matter and energy? Λ 74%

From Lineweaver (1998)

Value of 1 Ω Ω Λ 0.8 0.6 0.4 Ω CDM Why now? 0.2 0 0 1 2 3 Big Bang Present-day Age of the Universe

Hydrogen fusion fuelling a star s nuclear furnace E = mc 2

Hydrogen fusion fuelling a star s nuclear furnace

Why are we here?...

Why are we (and only we) here?... If the Universe is so big, and so old, and so finely balanced to support life like us, why are we seemingly alone?...

Why are we (and only we) here?... If the Universe is so big, and so old, and so finely balanced to support life like us, why are we seemingly alone?... Where is everybody? Enrico Fermi, 1950

Where is Everybody? by Stephen Webb Fifty solutions to the Fermi Paradox and the problem of extraterrestrial life: o They are here o They exist but have not yet communicated o They do not exist

They are here and are meddling in Human affairs

They exist but have not yet communicated We are Solar chauvinists Perhaps there are simply too many other interesting places to visit? Luminosity (Sun=1) 10 6 10 4 10 2 1 10-2 10-4 Surface temperature (K) 25000 10000 8000 6000 5000 4000 3000... Deneb..... Rigel........ Betelgeuse.... Antares.................. Arcturus Regulus Vega.......... Sirius A............. Mira Pollux Procyon A.. Altair. Sun............. Sirius B....... Procyon B................... -10 O5 B0 A0 F0 G0 K0 M0 M8 Spectral Type.. Aldebaran Barnard s Star. -5 0 +5 +10 +15 Absolute Magnitude

They exist but have not yet communicated Cloudy skies are common!

They exist but have not yet communicated They stay at home

They exist but have not yet communicated They stay at home and surf the net

They exist but have not yet communicated They are signalling but we don t know how to listen The Waterhole : strong H and OH emission between 1.42 GHz and 1.64 GHz

They do not exist Perhaps emergence of Life depends on astrophysics o e.g. carbon-based life may appear only after peak in cosmic carbon production o This peak occurred ~ 7 billion years ago. o Still leaves a ~ 3 billion-year head start for some ETC o Shouldn t that give plenty of time to colonise the galaxy?...

They do not exist Rocky planets are rare Most exoplanets found to date are hot Jupiters : massive planets close to their parent star. This is largely a selection effect.

Launch: Feb 2009 Kepler

They do not exist Jupiters are rare

Why are we (and only we) here?... If the Universe is so big, and so old, and so finely balanced to support life like us, why are we seemingly alone?...

Why are we here?... Selection effects can address the fine-tuning of our location within the Universe. But what about the global properties of the Universe?... The constants of nature The laws of nature

The Anthropic Principle Introduced by Brandon Carter (1973). A reaction to over-zealous use of the Copernican Principle: we are not at a special position in the Universe. "Although our situation is not necessarily central, it is inevitably privileged to some extent"

Why are we here?... Weak A.P. Selection effects can address the fine-tuning of our location within the Universe. But what about the global properties of the Universe?... Strong A.P. The constants of nature The laws of nature

The W.A.P. merely summarises the idea of selection effects for different locations in our Universe. The S.A.P. requires the existence of other universes with different constants / laws of nature.

There is a wide spectrum of views on the Anthropic Principle: Freeman Dyson: I do not feel like an alien in this universe. The more I examine the universe and the details of its architecture, the more evidence I find that the universe in some sense must have known we were coming.

There is a wide spectrum of views on the Anthropic Principle: Heinz Pagels: The influence of the anthropic principle on contemporary cosmological models has been sterile. It has explained nothing and it has even had a negative influence. I would opt for rejecting the anthropic principle as needless clutter in the conceptual repertoire of science.

There is a wide spectrum of views on the Anthropic Principle: Brandon Carter: The anthropic principle is a middle ground between the primitive anthropocentrism of the pre- Copernican age and the equally unjustifiable antithesis that no place or time in the universe can be privileged in any way.

Three ways to make a Multiverse: 1. Cosmological inflation 2. M-theory / String Landscape 3. Quantum cosmology

Present Day Big Bang

String Theory Point particles replaced by string loops Avoids infinities No unique theory (Branes in higher dimensions) Particle representation String representation

M-Theory More than the 3 spatial dimensions which we actually observe. Our observed spacetime emerges from the way in which the extra dimensions are compactified. Calabi-Yau manifold There could be up to 10 500 different compactifications each with a different set of constants of nature. We find ourselves in one that is anthropically selected.

Three ways to make a Multiverse: 1. Cosmological inflation 2. M-theory / String Landscape 3. Quantum cosmology

Present Day Big Bang

Summary of QM Interpretations Copenhagen Many Worlds Transactional

But is it science?... We can t observe the other universes. How do we test theories? How do we make predictions?... Much heated debate on these points.

Steven Weinberg Thomas Bayes (1702 1761 AD)

Ockham s Razor Frustra fit per plura, quod fieri potest per pauciora. It is vain to do with more what can be done with less. William of Ockham (1288 1348 AD) Everything else being equal, we favour models which are simple. How do we measure the simplicity of universe versus multiverse models?

Martin Gardner There is not the slightest shred of evidence that there is any universe other than the one we are in. No multiverse theory has so far provided a prediction that can be tested. As far as we can tell, universes are not as plentiful as even two blackberries

Evolving constants of nature?... Lee Smolin Paul Davies Cosmological Natural Selection?

But why do we live in this multiverse?... Hierarchical picture Different constants of nature Different laws of nature Different mathematical and logical structures