Category Archives: Yleinen

Paper: The Sibelius Project – E pluribus Unum

We introduce “Simulations Beyond The Local Universe” (SIBELIUS), a set of cosmological constraint simulations, that connect the Local Group to its cosmic environment. We show that introducing hierarchical small-scale perturbations to a density field constrained on large scales by observations provides an efficient way to explore the sample space of Local Group analogues.

Four Local Group realisations, created by varying intermediate scales. Being set on larger scales, the total mass, total energy and orientation are nearly identical. However, the separation of the two main haloes varies greatly between individual realisations.

Four Local Group realisations of the Sibelius project, created by varying intermediate scales. Being set on larger scales, the total mass, total energy and orientation are nearly identical. However, the separation of the two main haloes varies greatly between individual realisations.

Using the methods described in our previous paper, we create more than 60,000 simulations to identity we identify a hierarchy of Local Group characteristics emanating from different scales. We find that the total mass, orientation, orbital energy and the angular momentum are largely determined by modes above λ = 1.6 comoving Mpc (cMpc) in the primordial density field.

Smaller scale variations are mostly manifest as perturbations to the Milky Way – M31 orbit, and we find that the observables commonly used to describe the Local Group — the Milky Way – M31 separation and radial velocity — are transient, and depend on specifying very small scales down to 0.2 cMpc in the primordial density field.

View of a Local Group analogue from the Sibelius Project

Detailed view of a Local Group analogue from a dark matter simulation of the Sibelius project. The two large haloes are analogues of the Milky Way and M31.

We also find that the presence of M33/LMC analogues significantly affects the Milky Way – M31 orbit and its sensitivity to small-scale perturbations. We construct initial conditions that lead to the formation of a Local Group whose primary observables precisely match the current observations.

Paper: Setting the Stage – Structure Formation from Gaussian Random Fields

Cosmological simulations evolve a density field, specified at high redshift, forward in time. One of the tenets of the Inflationary Model is that the initial density fluctuations were created by quantum fluctuations within a fraction of a second during the Big Bang. Any density field can be described by the amplitudes of the fluctuations (the Power spectrum), which are determined by the cosmological model, and the phase information, which determines the specific peaks and dips in the density at different locations. A common way to parameterise this information is using Fourier modes.

In this paper, we use a new method for setting the initial conditions for cosmological simulations, building on earlier work by Adrian Jenkins, to specify the phase information. Instead of Fourier modes, we use orthogonal Octree basis functions, which have the benefit of being localised in space. By creating many variations of density fields, changing the phase information on different levels of the Octree (different spatial scales), we measure which scales in the very early universe effect the foŕmation of haloes we can observe today.

Dark matter density (left panel), an the difference in dark matter density in the same regions, when variations are introduced at different levels of the Octree.

Dark matter density (left panel), an the difference in dark matter density in the same regions, when variations are introduced at different levels of the Octree. Ast the scale of variations in the initial conditions increases from left to right, the difference in the final dark matter density grows. Smaller haloes disappear (and new ones appear), while larger haloes change in mass and position.

We quantify, for example, on what scales the information for objects like the Milky Way or the Virgo Galaxy cluster is defined. We also quantify how perturbations of the density field on smaller scales affect observable properties of the system, such as the mass, concentration, or position.

The level of the Octree (top axis) and the equivalent length scale (bottom axis) at which the density field needs to be specified in order for haloes of different masses to be uniquely defined. For example, Milky Way mass haloes (green line) are defined by density fluctuations of approximately 1.6 Mpc.

Beyond the theoretical interest, this method of setting the initial conditions, and varying the density field has important practical applications. By iteratively randomising the phase information, we can explore the sensitivity of observables to the initial conditions, and create objects that closely match observations – all while preserving the Gaussian nature of the initial density field created by the quantum fluctuations of the Big Bang.

Statement by Finnish Astronomers and Astrophysicists on Harassment

We, astronomers and astrophysicists from Finland and in Finland, strongly condemn harassment and discrimination. This includes but is not limited to harassment or discrimination based on sex, gender, sexual orientation, race, or disability. Harassment can take the form of unwanted sexual attention, bullying, coercion, or the creation of an unsafe or hostile work environment, especially in the presence of imbalances of power. Our own academic community is no exception.

Harassment is a serious offence that too often goes unreported and unchallenged. When  victims come forward, they must be able to rely on our support. We must address the issue head-on. Otherwise, we not only enable harassers, but also send a devastating message to the individuals who have been harassed – and whose careers are often destroyed or seriously disrupted – as well as to the whole community.

Our concern and solidarity is first with victims of harassment, and with the right of all staff and students to work in a healthy and safe environment. And while we also recognise the possibility of rehabilitation, it can only be at the end of a process that begins with an acknowledgement of the offense, and taking responsibility for the harm caused.

The Finnish astronomical and astrophysical community is diverse and international, and it is also deeply connected. It strives on principles of fairness and equal opportunities. Harassment or discrimination threaten our community and our way of working together. They have no place here.

Original statement online:

Finnish or Finland-based astronomers or astrophysicists, please use this form to sign:

Other academics, please use this form to show your support:

The APOSTLE collaboration

The APOSTLE collaboration is “A Project Of Simulating The Local Environment”.

An offshoot of the EAGLE collaboration, it was started by myself, Carlos Frenk, Azadeh Fattahi and Julio Navarro in 2013, and has now grown to a large international collaboration. To date, more than 20 papers based directly on APOSTLE data have been written by more than 30 different co-authors from Argentina, Belgium, Canada, Chile, China, Finland, Germany, Iran, Ireland, Italy, Mexico, the Netherlands, Poland, Romania, Switzerland, the UK and the US.

Click below for a list of APOSTLE papers published to date. We also keep a repository for ongoing projects (password required). If you are interested in using APOSTLE data for your own project, please send me an email (till+DOT+sawala+AT+helsinki+DOT+fi)

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Jean Sibelius 1913

Jean Sibelius by Daniel Nyblin, [Public domain], via Wikimedia Commons

Music to our ears: CSC, the Finnish national supercomputing centre, have approved our Grand Challenge application for the SIBELIUS project: simulations beyond the Local Universe. Lots of exciting things to come!