Sky and Telescope - June 2015 - 25

1995: R. WILLIAMS / HDT TEAM (STSCI) / NASA; 1998: R. WILLIAMS (STSCI) / HDF-S TEAM / NASA; 2004: NASA / ESA / R. WINDHORST (ARIZONA STATE UNIV.) / H. YAN (SPITZER SCIENCE CENTER / CALTECH); 2009: NASA / ESA / G. ILLINGWORTH
(UCO / LICK / UC SANTA CRUZ) / R. BOUWENS (UCO / LICK / LEIDEN UNIV.) / HUDF09 TEAM; 2012: NASA / ESA / G. ILLINGWORTH, D. MAGEE, AND P. OESCH (UC SANTA CRUZ) / R. BOUWENS (LEIDEN UNIV.) / HUDF09 TEAM

Facing page, far left: The Cepheid RS Puppis illuminates its
surrounding nebula. Center: Hubble recorded the shockwave
from SN 1987A slamming into a previously ejected shell of
material. Near left: These 1996 images reveal quasars' host galaxies, some of which (but not all) are colliding with other galaxies. Such images made explaining what triggers quasars more
complicated. Above: Hubble took five deep fields: Hubble Deep
Field (1995), Hubble Deep Field South (1998), Hubble Ultra
Deep Field (2004 and 2009), and Extreme Deep Field (2012).

5. Deep Fields
Arguably one of Hubble's most exciting results was
obtained when the space telescope was trained on a tiny
patch of "empty" sky in the Big Dipper in December 1995.
A number of influential astronomers were against the
idea (it would use up a lot of valuable observing time), but
a few thousand faint galaxies showed up, many of them
so remote that their light has taken more than ten billion
years to reach us. In a sense, the space telescope was used
as a time machine to provide cosmologists with a view of
how the universe looked in its infancy.
Over the past 20 years, Hubble has taken several
other "deep fields," in different parts of the sky, with
more sensitive cameras, and in a wider variety of wavelength bands, culminating in the Extreme Deep Field
and the Frontier Fields project (S&T: Jan. 2015, p. 20).
Looking back to a few billion years after the Big Bang
reveals irregular, clumpy galaxies smaller than today's,
the primordial building blocks of the mature galaxies
that currently populate the cosmos.
Hubble's infrared cameras play an important role in
this field of cosmic archaeology: during its multi-billionyear-trip to Earth, the energetic ultraviolet light emitted
by newborn stars in the very earliest galaxies is stretched
all the way into the infrared by the expansion of the
universe. As a result, some of the building blocks aren't
even visible at optical wavelengths.
Deep-field studies revealed the history of star formation in the universe, which peaked some 11 billion years
ago and has dwindled by a factor of 30 or so ever since.
But Hubble's unique windows into the past also have
posed puzzling questions about the unexpectedly fast
growth rate of galaxies and their central black holes in
the universe's first two billion years.

6. The Dark Side
The Hubble Space Telescope was named after pioneering cosmologist Edwin P. Hubble, co-discoverer of the
expanding universe. Indeed, the space telescope's most
important Key Project was to determine the age and
expansion history of the cosmos, by carefully calibrating
the astronomical distance scale. Before the space telescope, astronomers only knew that the Hubble parameter - a measure of the current expansion rate - was
somewhere between 50 and 100 km/s per megaparsec
(a parsec is about 3.26 light-years). Hubble narrowed in
on just above 70 km/s/Mpc, although the results don't perfectly match the rate calculated from observations of the
cosmic microwave background, which have pinned down
the age of the universe at 13.8 billion years and the current
expansion rate at 68 km/s/Mpc.
But Hubble's impact on cosmology turned out to be
much larger than anyone could have foreseen. Observations of colliding clusters like the famous Bullet Cluster
provided additional evidence for the existence of a mysterious "dark matter" in the universe - the data on galaxy
dynamics and mass distribution within the clusters are
hard to explain solely by the alternative gravity theory of
Modified Newtonian Dynamics (MOND), which tries to
explain the observations without unseen matter. Moreover, using the technique of weak gravitational lensing,
where the shapes of thousands of background galaxies
are slightly distorted by the gravity of intervening matter, astronomers are able to map out the distribution of
dark matter, sometimes even in 3D.
Finally, Hubble played a key role in the study of distant Type Ia supernovae that led to the discovery, in 1998,
of the accelerated expansion of the universe - one of the
most intriguing astronomical finds of the past century.
Apparently, another mysterious cosmic component,
dubbed dark energy, is pushing empty space away from
itself ever faster and faster.

7. The People's Telescope
There's one more way in which Hubble has forever
changed the face of astronomy. In a very direct sense,
it brought the universe into the living room - and into
the classroom, for that matter. Largely thanks to the
space telescope's stunning imagery, Hubble discoverSk yandTelescope.com June 2015

25


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Sky and Telescope - June 2015

Table of Contents for the Digital Edition of Sky and Telescope - June 2015

Contents
Sky and Telescope - June 2015 - Cover1
Sky and Telescope - June 2015 - Cover2
Sky and Telescope - June 2015 - 3
Sky and Telescope - June 2015 - Contents
Sky and Telescope - June 2015 - 5
Sky and Telescope - June 2015 - A
Sky and Telescope - June 2015 - B
Sky and Telescope - June 2015 - 6
Sky and Telescope - June 2015 - 7
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Sky and Telescope - June 2015 - Cover3
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