New Scientist – August 17, 2019

12 | New Scientist | 17 August 2019

FAST radio bursts could be the

key to a greater understanding

of the cosmos. If we can use

these milliseconds-long bursts

of radio waves to measure

distances across space, it

may help us figure out the

true nature of dark energy.

We measure cosmic distances

using objects with a predictable

brightness, or luminosity. By

comparing how these “standard

candles” appear from Earth

to how bright we know they

actually ought to be, we can

work out how far away they are.

There are only a few types

of standard candles, including

supernovae and stars with

well-known luminosities.

Now, Tetsuya Hashimoto at the

National Tsing Hua University

in Taiwan and his colleagues

have found a property of fast

radio bursts (FRBs) that may

let us use them in a similar way.

“The standard measures

that we have all have their

limitations,” says Amanda

Weltman at the University

of Cape Town in South Africa.

“If we can find a way to use

FRBs, then we get a new way

of constraining cosmology

that should be independent

of all the others.”

We have found dozens of

FRBs so far and expect to find

thousands in coming years,

including some further away

than the other standard candles.

This could make the FRBs

particularly useful, she says.

Hashimoto and his team

compared the luminosity and

duration of a sample of 27 FRBs

and found a correlation: the

longer a burst lasted, the

brighter it was (arxiv.org/

abs/1907.11730v1). So

measuring the duration of

a burst might allow us to

estimate its intrinsic brightness

and calculate its distance.

Although the luminosities

and durations didn’t all match

up perfectly, the researchers

calculated that there is less

than a 4 per cent probability

that this relationship is just

due to chance.

If this is confirmed and FRBs

can be used as standard candles,

they may help us figure out the

nature of dark energy, which is

speeding up the expansion of

the universe. If we know how far

an object is from Earth, we can

calculate how fast it is moving

away from us, which depends

on the universe’s expansion.

Spotting more FRBs from further

away could help us find out how

the expansion rate has changed

over the universe’s history.

It could also give us a clue

about what creates FRBs, which

are of unknown origin, says

Hashimoto. This idea that the

longer a burst is, the brighter it

will be supports three models

of what causes these signals:

the interaction of a black hole’s

powerful jet with a cloud of

debris; a collision between a

neutron star and a comet or

asteroid; and a shock travelling

through a supernova remnant.

It is unclear which, if any,

of these models is most likely,

and before FRBs are used widely

as standard candles, we will

have to figure out what causes

them. “The assumption that’s

going into this is that there’s

only one type of FRB, and that

seems unlikely,” says Weltman.

“We really need to know how

FRBs work before we can trust

this relationship.” ❚

A TECHNIQUE that uses trees

to spot minerals in the ground

has had one of its first major

successes, after a company struck

gold in South Australia.

Trees act as pumps, bringing up

mineral-containing water from

deep underground. By analysing

their leaves, it is possible to see

if gold is present below.

This is hard to do because the

quantity of such minerals in

leaves is tiny. But Australian firm

Marmota has now discovered a

vein of gold using the technique.

It is 6 metres thick with 3.4 grams

of gold per tonne. Although that

isn’t especially large, it could be

the tip of a new, larger deposit,

since the vein is 450 metres from

any previously known gold.

The firm had tested the

technique before, but this is

the first time it has been used

for genuine prospecting.

The find was made after leaves

collected from senna trees in the

area showed high levels of gold.

Firm uses trees to

prospect for gold

Minerals Cosmology

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Drilling confirmed its presence.

“For each new area, we need

to work out what plants are

present on site, which part of the

plant – leaves, twigs, bark etc. – is

likely to give the best response,

and potentially adjust for any

seasonal or local factors,” says

Aaron Brown at Marmota.

The work was aided by new

tools that can detect tiny traces of

minerals, such as coupled plasma

mass spectrometry. This uses a

plasma at 10,000°C to break a

sample into atoms for analysis.

Gold prospecting usually

involves collecting soil samples.

The new approach allows

sampling to occur without any

digging and sees much further

underground. “It is also a relatively

cheap method for a first pass

across an area,” says Nathan Reid

at the Commonwealth Scientific

and Industrial Research

Organisation in Australia.

However, when a significant

find is made, the subsequent

mining activity will be extremely

damaging to the environment. ❚

Trees suck up water from

below that contains

minerals, such as gold

6 metres

The thickness of a vein of gold

found by testing tree leaves

“ We expect to find

thousands more

fast radio bursts in

the coming years”

Odd space signals

could unmask

dark energy