Astronomy - Top Discoveries from Space Telescopes.pdf

(10466 KB) Pobierz

Top seven

science

DIS

form without permission from the publisher.

www.Astronomy.com

Great telescope triumphs

Galaxies like the beautiful

Whirlpool (M51) played a key

role in many of Hubble’s great-

est discoveries. Such island

universes are held together by

dark matter, harbor supermas-

sive black holes at their cen-

ters, help define the Hubble

constant, and fly away from

one another at increasing

rates thanks to dark energy.

NASA/ESA/S. BECKWITH (STS

I)/THE HUBBLE

HERITAGE TEAM (STS

I/AURA)

From the incandescent brilliance

of the first stars and galaxies to the

overwhelming power of dark matter

and energy, the space telescope has

illuminated many cosmic mysteries.

by Mario Livio

SCOVERIES

W W W.AS TR ONO MY.CO M

Space shuttle astronauts saw the

Hubble Space Telescope for the

last time in 2009 when they bid

it farewell following the last ser-

vicing mission.

NASA; BACKGROUND IMAGE

OF NGC 265: NASA/ESA

ew scientific experiments have

enjoyed 25 years of relentless pro-

ductivity and a continuous stream of

discoveries. Yet this is only one of the

Hubble Space Telescope’s remarkable

achievements. Not only have Hubble obser-

vations transformed our ideas about almost

every topic in astronomy and astrophysics, but

the drama associated with space shuttle astronauts servicing the

observatory and the impact the telescope has had on the public’s

appreciation of science also have made Hubble unique in the

history of science.

Hubble’s scientific successes are so numerous that it is not easy

to select its greatest hits. Consequently, the list on the following

pages represents my own personal biases as to what the telescope’s

most important contributions have been.

I also should emphasize that it is rare in astronomy to be able

to attribute a discovery to one particular observation or a specific

observatory. More often, progress in understanding phenomena

results from a series of observations at different wavelengths by

a variety of telescopes over a long period of time. I do not claim,

therefore, that Hubble acted alone in making these seven discover-

ies. Instead, I chose findings in which space telescope observations

played a crucial role.

In selecting Hubble’s most important breakthroughs, I was

guided by two principles: Either the discovery had to contribute

significantly to our understanding of the universe as a whole, or

it had to represent a major step in the quest to determine whether

extraterrestrial life exists. The second point arguably ranks as one

of the most intriguing pursuits in science today.

Mario Livio

is a senior astrophysicist at the Space Telescope Science

Institute in Baltimore, which conducts the scientific program of Hubble. His

most recent book is

Brilliant Blunders

(Simon and Schuster, 2013).

AS TR ON OM Y

•

A P RI L 2015

The gravitational attraction

of all the matter in the uni-

verse should cause cosmic

expansion to slow down.

But in 1998, two groups

of astronomers discovered

the exact opposite: The rate

of universal expansion is

accelerating. The research-

Supernova 1994D (lower left) in galaxy

NGC 4526 helped astronomers pin down

ers based their discovery

the universe’s accelerating expansion,

on observations of stellar

which is powered by a repulsive force

explosions known as type

exerted by dark energy.

NASA/ESA/THE HUBBLE

Ia supernovae, which occur

KEY PROJECT TEAM/THE HIGH-Z SUPERNOVA SEARCH TEAM

when white dwarf stars grow

to their limiting mass of

about 1.4 solar masses. Only Hubble could view the most distant

of these explosions and thus confirm the acceleration.

All current studies indicate that a still-mysterious form of

energy, dubbed dark energy, propels this speed-up. Although sci-

entists do not yet understand the precise nature of dark energy,

they have deduced some of its properties. These efforts suggest

that it is the energy associated with empty space, or what scientists

call the physical vacuum.

That the vacuum contains energy is not surprising in itself.

Quantum mechanics — the physics that describes the universe

at the smallest scales — predicts that the physical vacuum is far

from empty. Instead, it teems with virtual pairs of particles and

Dark energy

Water megamasers — amplified microwave emissions from water molecules — orbiting the central supermassive black hole in spiral galaxy M106 pinned

down this object’s distance. Hubble’s calibration of Cepheid variable stars in the galaxy played a crucial role in determining the Hubble constant.

antiparticles that appear and disappear within tiny fractions of a

second. The problem has been that every theoretical attempt to

calculate what the energy density of the vacuum should be has

missed the target by several orders of magnitude.

Given the quickening expansion, what will the fate of our uni-

verse look like in the distant future? If dark energy does represent

the energy of empty space, which has a constant

density, then the expansion will continue to accel-

erate. About a trillion years from now, astrono-

mers living in the merged product of the Milky

Way and the Andromeda Galaxy — the two are

expected to collide about 4 billion years from now

(another Hubble result, by the way) — will not be

able to see any other galaxy. The universe then will

be well on its way toward a cold death.

The Hubble constant

and the universe’s age

Ever since the 1920s and the seminal works of

astronomers Vesto Slipher, Georges Lemaître, and

Edwin Hubble, scientists have known that the uni-

verse is expanding. The so-called Hubble constant

is a measure of the current expansion rate, and its value is inverse-

ly proportional to the age of the universe. Until the space tele-

scope’s launch, published values of the Hubble constant differed

by as much as a factor of two. One large group of astronomers

claimed a value around 50 kilometers per second per megaparsec,

while the other main faction declared a rate near 100 km/s/Mpc.

Few scientific

experiments have

enjoyed 25 years

of relentless

productivity and

a continuous

stream of

discoveries.

And each side in the argument insisted that their data supported

an improbable error of only about 10 percent.

One of Hubble’s “key projects” was to resolve this conundrum.

Using the space telescope’s superb optical resolution, the key project

team examined a number of distance indicators, including Cepheid

variable stars, the Tully-Fisher relation that links a spiral galaxy’s

rotation rate to its intrinsic luminosity, and type

Ia supernovae. By 2001, the team refined the

Hubble constant’s value to 72 km/s/Mpc with

a precision of about 10 percent.

A combination of this new value with the

discovery of cosmic acceleration and a new

assessment of the ages of globular star clusters

resolved yet another mystery — the universe is

indeed older than its oldest known stars. For

cosmologists who believed in a Hubble constant

of 100 km/s/Mpc, a simple calculation shows

that the universe would be only about 10 billion

years old, yet the ancient stars in globular clus-

ters appeared to be at least 12 billion years old.

According to the most recent determination of

cosmological parameters by the European Space

Agency’s Planck satellite, the universe is 13.8 billion years old with

an uncertainty of just 40 million years.

But scientists have not rested on their laurels. Thanks largely to

further Hubble observations, in the past decade astronomers have

made impressive progress in measuring the Hubble constant more

precisely. By cross-calibrating several distance indicators — such

W W W.AS TR ONO MY.CO M

NASA/ESA/THE HUBBLE HERITAGE TEAM (STS

I/AURA)/R. GENDLER (FOR THE HUBBLE HERITAGE TEAM)

Each deep field exposed thousands of

galaxies in an area of sky you would

see looking through a drinking straw.

cosmos appears to be homogenous and isotropic — the same at

every location and in every direction — these findings imply that

the observable universe holds a few hundred billion galaxies.

The deep observations have provided astronomers with a

treasure-trove of data about galaxy evolution. One key result has

been learning the cosmic star-formation rate — how fast the uni-

verse as a whole creates new stars as a function of distance, or cos-

mic time. (See “How fast do stars form?” below.) Knowing how

quickly stellar mass builds up in galaxies provides fundamental

constraints on models of how galaxies form and evolve.

Black holes at the centers of galaxies

The Hubble Deep Field South seen here is one of a series of observations

astronomers made with Hubble that helped establish the cosmic star-

formation rate.

R. WILLIAMS (STS

I)/THE HDF-S TEAM/NASA/ESA

The cosmic star-formation rate

Cosmic star-formation rate

(Solar masses/year/Mpc

)

as Cepheid variables, type Ia supernovae, and the amplified micro-

wave emissions from water molecules (so-called megamasers) in

orbit around the supermassive black hole at the center of galaxy

M106 — they reduced the uncertainty in the Hubble constant’s

value to about 5 percent by 2009 and to 3 percent by 2011.

Currently, researchers are using new scanning techniques with

Hubble’s Wide Field Camera 3 to get even more precise distances

to Cepheids in the Milky Way at distances of about 3,000 to 10,000

light-years. These promise to shrink the Hubble constant’s uncer-

tainty to just 2 percent. And the European Space Agency’s ongoing

Gaia mission should make robust progress by extending accurate

Cepheid observations out to 33,000 light-years.

Refining the Hubble constant in the relatively nearby universe

to a precision of 1 percent may help resolve the latest apparent

tension between different measurements. The so-called local

value currently stands at about 73 km/s/Mpc, while that inferred

from Planck observations of the distant universe holds at about

68 km/s/Mpc. The discrepancy may reflect only higher systematic

errors than anyone suspects, but if it turns out to be real, it hints at

some potentially new physics.

Some of Hubble’s most dramatic observations have been long-

exposure photographs of what, at first blush anyway, appeared to

be rather bland areas. Starting with the original Hubble Deep Field

— an observation made of a tiny region in the constellation Ursa

Major over 10 days in December 1995 — the space telescope has

carried out several deep observations of small patches of sky.

These programs have revealed just how small our physical exis-

tence is in the grand cosmic scheme.

Each deep field exposed thousands of galaxies in an area of sky

you would see looking through a drinking straw. Given that the

AS TR ON OM Y

•

A P RI L 2015

Even before Hubble opened its eyes to the universe, observations

indicated that at least some galaxies harbor supermassive black

holes in their cores. And theoretical models of active galaxies and

of quasars — extraordinarily energetic point-like objects in the

distant universe — suggested that matter accreting onto such black

holes from their surroundings powered their emissions. Hubble

observations turned these hints and tentative ideas into certainty.

The space telescope has shown that essentially every galaxy

that has a bulge of stars at its center hosts a supermassive black

hole. These black holes range in mass from perhaps as low as a

few tens of thousands of times the Sun’s mass in dwarf galaxies

to a few billion solar masses in massive galaxies. Hubble also has

directly imaged the host galaxies of a few quasars, demonstrating

unambiguously that the engines driving these objects reside at

the centers of galaxies.

Most significantly, however, Hubble observations revealed a

fairly tight correlation between the relative speeds of the stars in

the galaxy’s central bulge (what astronomers call the velocity dis-

persion) and the mass of the black hole. The velocity dispersion,

in turn, depends on the mass of the bulge.

How fast do stars form?

0.1

Best ﬁt

Ultraviolet

Ultraviolet + infrared

Hydrogen-alpha

Infrared/far-infrared

1.4 gigahertz

0.01

Redshift (z)

The cosmic star-formation rate peaked at a redshift of around 2

(which corresponds to a look-back time of 10 billion years). Today’s

universe produces only 0.01 solar mass of stars per year in a typical

cubic megaparsec (Mpc), which is approximately 35 million cubic

light-years.

ASTRONOMY:

ROEN KELLY, AFTER P. S. BEHROOZI, R. H. WECHSLER, AND C. CONROY (A

770, 57)

Plik z chomika:

FatumTech

Astronomy - Top Discoveries from Space Telescopes.pdf

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: