http://www.skyandtelescope.com.au 41the expansion rate throughout cosmic history will help
astronomers discover whether or not dark energy has changed
over time. If the rate is stable, then a model that relies on
the cosmological constant — which suggests that dark energy
arises from the short-lived virtual particles and antiparticles
within empty space — might be the best explanation. But if
it does change, then a model that relies on quintessence —
which suggests that dark energy results not from the vacuum
of space but from a field that pervades spacetime — might be
more appropriate.
A better understanding of Type Ia supernovae will help
scientists pin down not only dark energy and our cosmic past
but also the fate of our universe, which is determined by dark
energy’s true nature.
Filippenko argues that one way forward is to study the
extreme cases. Consider a lesser-known class dubbed Type
Iax. These explosions look like Type Ia supernovae but move
slower, fade faster and have less energy overall. You can
almost think of them as a miniature stellar blast.
“They’re sort of like this different beast,” Foley affirms.
“But they’re similar enough that we know some of the
physics has to be shared.”
The data scientists have gathered suggest that Type
Iax supernovae occur in single-degenerate systems. But
in this situtation, the companion star has already lost its
outer layer of hydrogen, meaning that the white dwarf
accumulates helium instead. Jha speculates that perhaps
that’s the difference between normal Type Ia supernovae
and Type Iax supernovae — one accretes hydrogen and the
other helium.
Unfortunately, scientists have very few examples of oddball
Type Ia supernovae. So they’re scouring the skies in search of
every system that might soon explode.
Among those efforts is the Zwicky Transient Facility in
California, which reached first light in late 2017. Like its
predecessor, the Palomar Transient Factory (which caught
both of the game-changing supernovae in 2011), the ZTFi tl l t t i t’s
bright
Hemi
Chile ae
overitslifetime.
Butthekeyisn’tsimplytofindnewsupernovae;it’sto
catchthemearlyandthenobservethemoften.Inthepast,
supernovaewereoftenimagedonceevery3 or4 days— time
gapsthatmeantastronomersmightmisscrucialdetails.But
observatoriesliketheTransitingExoplanetSurveySatellite
enableastronomerstoobservetheseexplosionsevery 30
minutes.Follow-upwillbedonefromtheground,including
bythoseinAndrewHowell’sGlobalSupernovaProject,a
collaborationof 150 astronomersaroundtheworldwho
willgatherlightcurvesandspectrawhensupernovaeare
discovered.
Withsomanysurveysonlineorintheworks,
astronomersareoptimisticthatthey’llsoonhaveenough
datatopindownwhatsetstheseexplosionsinmotionand
howoften.“Nosupernovacanhidefromusnow,”Nugent
says.“We’regoingtofindit duringthenightorthenext
nighteverysingletime.”SHANNONHALL’Sfavouritesupernovaremnantis theCrab
EXPLOSION GRAPHIC: TATIANAZAETS / ISTOCK / GETTY IMAGES PLUS Nebula,producedbya starthatexplodedin AD1054.
B C
nse, naked cores of dead s
hed its outer layers. They’r
ygen. Many have outer helium env
a thin layer of hydrogen. Some sho
t when they explode, there’s usually
elium in their spectra — that’s one
stinguish Type Ia supernovae from
stellar explosions.